9Camera Features

Advances in camera design are due to improvements in specific features of camera mechanisms, leading to the emergence of a few popular types of camera. Owing to the complexity of each of these features, many of which are interdependent, it is helpful to discuss each in detail, including systems for the shutter, viewfinder, focusing and exposure metering, as well as the iris diaphragm and flash synchronization. Camera lenses are detailed in Chapter 7.

Shutter Systems

Both the type of shutter and its operational range contribute significantly to the capabilities of a camera. The function of a shutter is to open on command and expose the sensitized material to the action of light for a predetermined time, as chosen by the user or by an automatic exposure-metering system. An ideal shutter would expose each part of the film equally and simultaneously, i.e. it would allow the cone of light from the lens aperture to the focal plane to fall upon the film for the entire duration of the exposure. It should be silent in operation; cause no jarring or vibration; and require little effort to set in motion. The effective exposure duration (‘shutter speed’) should be accurately repeatable. The perfect shutter has yet to be achieved.

There are two main types of shutter in general use, namely the leaf or between-lens shutter and the focal-plane shutter; both can be purely mechanical, or more usually combine both mechanical and electronic features.

Between-Lens Shutters

The ideal shutter position to control the light transmitted by a lens is near the iris diaphragm, where the beam of light is at its narrowest, and so the minimum amount of shutter blade travel is required to allow light through. The film gate area is also uniformly exposed at all stages in the operation of this type of shutter.

Simple cameras may use single- or double-bladed between-lens shutters, but others use multi-bladed shutters (see Figure 9.1). Usually five blades or sectors are used, and these open somewhat like the leaves of an iris diaphragm. Sometimes an ‘everset’ or ‘self-setting’ type of shutter mechanism is used, where a single control is depressed to compress the operating spring mechanism and then release the shutter in one movement. The speed range available may be limited to only a few settings.

Usually, between-lens shutters are of the ‘preset’ type where two movements are required: one for tensioning (‘cocking’) the operating spring and one for releasing the shutter. The tensioning operation is part of the film advance mechanism. Mechanical preset shutters of the Compur, Copal and Prontor types provide exposure times ranging from 1 s to1/500 s plus a ‘B’ setting, where the shutter remains open while the release button is held depressed to give time exposures. Occasionally a ‘T’ setting is available, where the shutter opens when the release is pressed, and remains open until the button is pressed again to close it. To operate, the shutter blades pivot about their ends (or, rarely, centre) and, for all but the shortest durations, open with the same velocity and acceleration. An additional spring mechanism may increase operating velocity at the highest speeds, while the slower speeds are controlled by engaging a gear train to retard the blade closure mechanism. The characteristics of leaf shutters are shown in Figure 9.2.

On older shutters, the series of shutter speeds was 1, 1/2, 1/5, 1/10, 1/25, 1/50, 1/100, 1/250 and 1/500 second. Modern shutters provide 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250 and 1/500 second, to give a progression of exposure increments similar to that provided by the standard series of lens aperture numbers, i.e. with each step double that of the previous one. This latter arrangement was originally designed to permit the introduction of an optional mechanical interlock between the aperture and shutter speed controls, in order to keep the two in reciprocal relationship with reference to an exposure value (EV) number. As the shutter speed was changed, the iris diaphragm was automatically opened or closed to keep exposure unchanged.

The effective exposure duration of a particular shutter-speed setting depends on variables such as the age and state of the shutter, the particular speed selected and the aperture in use. Calibration may be necessary for critical work. Shutter speeds are usually set by click stops on the selector control although the design of the shutter may in some (but not all) cases permit intermediate values to be set. For reasons of economy some shutters do not have speeds longer than 1/30 s. Large-diameter lenses as used in large-format cameras require shutters with larger, heavier blades, and the top speed may be limited to 1/250 s or longer.

Many between-lens shutters include electronic control as well as mechanical operation. Such electronic shutters may have the blades opened by a spring mechanism, but the closing mechanism is retarded by an electromagnet controlled by timing circuitry to give a range of shutter speeds. A typical resistor–capacitor timing circuit element is shown in Figure 9.3. Switch S is closed by the shutter blades opening, and battery B begins to charge capacitor C through a variable resistor R. The time taken to reach a critical voltage depends on the value of R, but when it is reached the capacitor discharges to operate a transistorized trigger circuit T which releases the electromagnet holding the shutter blades open. Changing the shutter speed is accomplished by switching in a different value of R. In automatic cameras the control unit may be a CdS photoresistor or silicon photodiode (SPD) arrangement which monitors the subject luminance and gives a continuously variable shutter speed range within the range available. More precise exposures may be given, rather than just using the marked values on the setting control. A visible or audible signal may give a warning when the required exposure time is longer than 1/30 s to indicate the need for a tripod, or change to electronic flash.

As the mechanism of an electronic shutter is still largely mechanical, there is no improvement on earlier designs in terms of performance at higher speeds or greater efficiency. It does however lend itself to automation and remote control. The camera body may carry the shutter speed control for a range of interchangeable lenses, each fitted with its own shutter and with means for interfacing with an accessory exposure-metering prism finder. In lenses for technical cameras a control box for selecting shutter speeds and apertures can be used on a long cable. Electronic shutters all require battery power for operation, but in case of failure, a single mechanical shutter speed may be available for emergency use. This is usually the lowest shutter speed that requires no electronically controlled retardation and still synchronizes with flash.

The shutter may have some further features incorporated. Besides the usual shutter speed and aperture scales, an interlocked shutter has a third scale of exposure values (EV). Typical EV scale values range from 2 to 18 or so, with a change from one number to the next corresponding to an alteration in the luminance of the subject by a factor of 2. The EV scale can be used with a suitably calibrated exposure meter. Also, a ‘self-timer’ or delayed-action device is often fitted; by means of this the release of the shutter can be delayed for some 5 to 15 s. Most shutters for technical cameras have a ‘press-focus’ button which opens the shutter on a time setting irrespective of the shutter speed preselected. This feature facilitates focusing on the ground-glass screen and eliminates the necessity for constant resetting of shutter speed to T or B for this operation.

A great practical advantages of between-lens shutters is the simplicity of flash synchronization at all shutter speeds. This enables the use of electronic flash for fill-in purposes, which is discussed fully in Chapter 20.

Focal-Plane Shutters

This type of shutter is located in the camera body and travels close to the film plane. In earlier forms, the focal-plane shutter consisted of an opaque blind with a slit of variable width, or of several fixed slits of various widths; the slit chosen was driven past the front surface of the film at a chosen velocity, progressively exposing the film as it passed across. With a focal-plane shutter the film area is exposed sequentially, unlike the action of a between-lens shutter. The effective exposure time (t) given to any portion of the film frame is given by the slit width (W) divided by the slit velocity (V).

Classic shutters of this type used two opaque blinds pulled across the film gate by spring action (see Figure 9.4). The first or leading blind starts to uncover the film when the shutter release is pressed and the second or trailing blind follows to cover up the film, with a greater or a smaller gap (slit) according to the delay set and timed by the shutter speed selected. The separation between the rear edge of the first blind and the front edge of the second blind gives a slit of adjustable width which allows a range of exposure durations. The slit may also vary in width during its travel to compensate for acceleration of the slit during the exposure. The shutter is also ‘self-capping’ as the slit is closed by overlap of the blinds when they are reset by operation of the film advance mechanism.

Focal-plane shutters originally used rubberized cloth blinds, but materials such as aluminium, titanium and polymers can be used. Modern designs use a slit formed by a vertically travelling fan of interlocked blades, as they can be made with the right combination of lightness and strength (Figure 9.5). The slit may travel either sideways or vertically with respect to the film gate. Vertical travel is preferred as flash synchronization speeds may be shorter due to the reduced travel time across the frame. When photographing rapidly moving subjects with a focal-plane shutter, sequential exposure to the optical image of the subject may result in distortion, depending on the direction of movement of the slit relative to that of the subject.

Factors associated with focal-plane shutter design have been those of uniform exposure across the gate, avoidance of shutter bounce, and flash synchronization. The slit width and velocity must be maintained over the whole film area, with the adjustments discussed above. For instance, an exposure of 1/2000 s demands an accurately maintained slit width of 1.5 mm between leading and trailing blinds travelling at some 3.6 metres per second, remembering the necessity for acceleration from and deceleration to rest positions. This action may be necessary up to five times or more per second during motorized operation. Figure 9.6 shows slit travel behaviour. Problems increase with format size and this, particularly with flash synchronization limitations, has resulted in some medium-format cameras favouring between-lens shutters, as used in large format cameras. Shutter bounce is indicated by a narrow strip of over-exposure at the edge of the film frame where the trailing blind has recoiled momentarily on cessation of its forward travel.

The typical exposure time range available can be from tens of seconds to 1/8000 s (0.125 ms) or less, but the actual transit time of the shutter may be from 1/250 s to as much as 1/30 s even at its minimum effective exposure duration. Flash synchronization for flash bulbs gave problems, as special focal plane (FP) bulbs with a long burning time (now unobtainable) were needed for the higher shutter speeds. The electronic flash equivalent is a flash unit with a special ‘strobed’ mode which gives an effective flash duration of some 20 to 40 ms, allowing electronic flash synchronization even at shutter speeds of 1/12 000 s.

Conventional electronic flash with its negligible firing delay and short duration poses other problems. The film frame must be fully uncovered at the instant the flash fires, i.e. when the leading blind has uncovered the film gate but before the trailing blind starts its travel. The flash is normally triggered by the leading blind when it has completed its run (see Figure 9.7). Depending on the shutter design and the format, the minimum shutter speed for synchronization is at a setting (usually marked ‘X’) of between 1/40 and 1/250 s. Shorter exposure durations cannot be used because the frame would only be partly exposed, the shutter aperture being narrower than the film gate. At suitable synchronization speeds, subject movement may be effectively arrested by the short flash duration, but double images or over-exposure may occur at high ambient light levels. Synchro-sun techniques are therefore restricted. Between-lens shutters are therefore preferred for more sophisticated flash work. The feature of ‘rear curtain synchronization’ triggers the flash just before the second blind starts to cover up the film gate and is usually used at the termination of a short time exposure when a combination of subject blur and sharp image is required.

Where the travelling slit is formed by an elaborate system of pairs of blades in a guillotine action, or by end-pivoted arrays of fan-shaped blades, use of a vertically travelling slit with a 24 × 36 mm format can give a 50 per cent reduction in slit travel time compared with horizontal travel, so flash synchronization is possible at higher shutter speeds, currently some 1/300 s.

Focal-plane shutters allow easy interchange of lenses on a camera body; the lenses do not need individual costly between-lens shutters; and a wider range of shutter speeds is available. Cameras that use interchangeable lenses equipped with their own between-lens shutters require a separate capping shutter within the camera body, to act as a light-tight baffle when focusing the camera or changing lenses.

Modular design of shutters allows easy replacement or repair when necessary. For many years mechanically operated shutters did not permit the automation of exposure; the user manually set the shutter speed at a pre-selected aperture in accordance with a separate or built-in (but uncoupled) light meter. The advent of electromechanically operated shutters equipped with electronic timing systems for release of the shutter blinds allowed integration of the shutter mechanism with the light-metering circuitry. Continuously variable exposure durations from many seconds to 0.25 ms could be given (though even mechanical shutters can have continuously variable exposure times). Hybrid shutters may use electronic timing for speeds longer than about 1/60 s and mechanical timing for shorter times, thus allowing the shutter to be used with electronic flash and a limited range of speeds in the event of battery failure.

Camera shutters are usually operated by a body release situated in an ergonomically designed position that reduces the possibility of camera shake and gives fingertip operation. The body release may also operate a reflex mirror, automatic diaphragm or exposure metering system by first pressure, before the shutter is actually released. Electronic shutters are operated by a release which is simply an electrical switch. Such devices readily lend themselves to remote control by electrical or radio impulses or from special accessory camera backs which incorporate intervalometers or automatic exposure-bracketing systems.

When used in conjuction with an aperture-priority automatic exposure mode, a camera body equipped with an electronically controlled focal-plane shutter may be attached to all manner of optical imaging devices, for which the effective aperture may not be known.

The Iris Diaphragm

The intensity of the light transmitted by a photographic lens is controlled by the iris diaphragm or stop which is usually an approximately circular aperture. In some simple cameras the aperture is fixed in size; in others the diaphragm consists of a rotatable disc bearing several circular apertures so that any of them may be brought in line with the lens. Such fixed apertures are known as Waterhouse stops and are used, for example, in some fish-eye lenses. This arrangement is limited in scope. Most lenses use an iris diaphragm, the leaves of which move to form an approximately circular aperture of continuously variable diameter. When the camera shutter is of the between-lens type the diaphragm is part of the shutter assembly. It is operated by a rotating ring, usually with click settings at half-stop intervals, and calibrated in the standard series of f-numbers. The interval between marked values will be constant if the diaphragm blades are designed to give such a scale; in older lenses with multi-bladed diaphragms the scale may be non-linear, and the smaller apertures cramped together (Figure 9.8).

The maximum aperture of a lens may not be in the conventional f-number series, but can be an intermediate value, e.g. f/3.5. The minimum aperture of lenses for small-format cameras is seldom less than f/16 or f/22, but for lenses on large-format cameras, minimum values of f/32 to f/64 are typical. The SLR camera requires automatic stopping down to the chosen aperture immediately before exposure, in order to permit viewing and focusing at full aperture up to the moment the shutter is released.

In early SLR cameras, the lens was stopped down manually by reference to the aperture scale. The use of click-stop settings assisted this process. Next, a pre-setting device was introduced which, by means of a twist on the aperture ring, stopped the lens down to the preset value and no further. This arrangement is still occasionally used, for example, on perspective control (PC) lenses. The next step was to introduce a spring mechanism into this type of diaphragm, triggered by the shutter release. The sequence was thus speeded up, but the spring needed resetting after each exposure and the diaphragm then re-opened to its maximum value. This was known as the semi-automatic diaphragm. Finally, with the advent of the instant-return mirror came the fully automatic diaphragm (FAD). In this system an actuating lever or similar device in the camera, operated by the shutter release, closes the diaphragm down during the shutter operation. On completion of the exposure the diaphragm re-opens and the mirror returns to permit full-aperture viewing again (Figure 9.9). A manual override may be fitted to allow depth-of-field estimation with the lens stopped down. Some cameras with early forms of through-the-lens (TTL) metering required the lens to be stopped down to the preset aperture for exposure measurement. Cameras with a shutter-priority mode have an additional setting on the lens aperture scale, usually marked ‘A’, where the necessary f-number is determined by the camera and set just before exposure, the user having selected an appropriate shutter speed. The aperture set may be in smaller increments than the usual half-stop increments, possibly to 1/10 stop if motorized and under digital control.

In most modes the TTL metering system in a camera normally requires the maximum aperture of the lens in use to be set into the metering system to allow full-aperture metering with increased sensitivity. Various linkages between lens and body are used for data transfer.

As focal length increases, the problems of fitting a mechanically actuated automatic diaphragm increase, so electrical systems are preferred. Some ultralong-focus lenses are not connected electrically and require manual setting of the aperture. Extension tubes usually transmit the actuation for diaphragm operation via pushrods or electrical connections, but extension bellows may not do so, and manual operation, or use of a double cable release, may be necessary.

Lenses fitted to large-format technical cameras are usually manually operated, although some presetting devices are available, as is electronic control from film plane metering systems. Some lenses have fixed apertures such as mirror lenses and those used in simple cameras.

Viewfinder Systems

Viewfinder Functions

The principal functions of the viewfinder are to indicate the limits of the field of view of the camera lens in use; to enable the user to select and compose the picture; to give a data display (Figure 9.10), and to assist in focus or exposure determination. Except for those fitted to simple cameras, viewfinders provide a method of focus indication, such as by a rangefinder, ground-glass screen or autofocus system. The type of viewfinder used often determines the shape and size of the camera, as with the TLR type, and the choice of a particular type of camera may even be related to the ease of use of the viewfinder, especially if the user wears spectacles. The viewfinder also acts as a ‘control centre’ and has a data display system primarily for the exposure measurement system, with a variety of indices, needles, icons, alphanumerics, lights and camera settings visible around or within the focusing screen area. There may even be either an ‘eye-start’ function where putting an eye to the viewfinder switches on the camera functions, or an interactive viewfinder where the direction of gaze activates different autofocus zones.

Simple Viewfinders

The simplest finders, as fitted to early box cameras and as supplementary finders for large-format cameras, employed a positive lens of about 25 mm focal length, a reflex mirror inclined at 45 degrees and a ground glass by which the image was viewed. This finder was used at waist level; the image illumination was poor. This early type was superseded by the ‘brilliant’ finder, which employed a second positive lens in place of the screen; this imaged the first lens in the plane of the viewer’s eyes, giving greatly improved luminance. Such finders are still occasionally found on simple cameras, and as viewfinders on exposure meters.

A versatile simple finder for use at eye level is the frame finder (Figure 9.11a). A metal open frame, in the same proportions as the film format, is viewed through a small peep-sight to define the subject area. This type of finder is still available as an accessory and is compact when collapsed. Refinements to the basic design give exact delineation of the subject area and parallax compensation. Use is now mainly confined to technical, aerial and underwater cameras.

Direct-Vision Optical Finders

Basic optics provide a direct-vision optical finder for use at eye level (Figure 9.11b). In the simplest type a strongly diverging negative lens is used to form an erect virtual image which is viewed through a peep-sight or weak positive eyepiece lens. This latter type, usually called a Newtonian finder, is in effect a reversed Galilean telescope (Figure 9.11c), the two lenses combining to produce a small bright erect virtual image.

A great improvement, the van Albada finder, has a mask bearing a white frame line in front of the positive lens: the negative lens has a partially reflecting rear surface. As a result, the white line is seen superimposed on the virtual image (Figure 9.12). The view through the finder extends beyond the frame line so that objects outside the scene can be seen. This is a great aid in composing, especially if the subject is moving. The eye may be moved laterally without altering the boundary of the scene. This finder is prone to flare and loss of the frame line in adverse lighting conditions; an improved version is the suspended frame finder, where a separate mask carrying one or several selectable frame lines for different lenses is superimposed on the field of view by a beamsplitter in the optical finder. The frame lines are separately illuminated by a frosted glass window adjacent to the finder objective lens (Figure 9.13). This type of finder has been considerably improved, and usually incorporates a central image from a coupled range-finder (Figure 9.14). Other refinements are compensation for viewfinder parallax error by movement of the frame line with the focusing mechanism, and a reduction in the frame area when focusing at closer distances. A through-the-view-finder (TTF) exposure metering system can also be incorporated. As seen in such finders, the image size is usually around × 0.7 to × 0.9 life-size. Simpler cameras with non-interchangeable lenses may have a finder giving a life-size image so that both eyes may remain open, giving the impression of a frame superimposed on the scene.

For technical cameras, a zoom-type Albada finder for use with a range of lenses is available, particularly for hand-held camera applications.

Kepler Type Real Image Finder

The reversed Galilean type of finder provides a bright, upright, unreversed and virtual image. It is not easy to make this finder very compact or to provide a zoom mode. Instead, another type of telescope is adapted, the Kepler telescope used in reverse mode. This gives a real image but it is inverted and laterally reversed, so requires additional optics in the form of a prism erector system (like binoculars). Various prism systems are used (Figure 9.15), including pentaprism, porro and Konig types. The light path can usefully be folded optically to fit inside a compact camera body. A very bright image is given and an intermediate lens gives a zoom action coupled to a zoom lens. It is not easy to provide parallax correction or field frame lines so the field of view is usually less than that of the lens to give a margin for error. A LCD plate in the system can display alternative format lines and some data. Aspherical plastic elements can reduce distortion and a beamsplitter can sample light for through the finder exposure metering.

Ground-Glass Screen Viewfinders

Most early cameras used a plain ground-glass screen upon which the image from the lens was composed and focused, and which was then replaced by a plate-holder or film-holder in order to make the exposure. This system is still used in technical cameras. The advantages of exact assessment of the subject area covered by the lens, accurate focusing and appraisal of the effects of camera movements offset any inconvenience in viewing an image that is both inverted and reversed.

Other cameras use a reflex system, with a front-surface mirror inclined at 45 degrees to the optical axis giving an image on a ground-glass screen in an equivalent focal plane. The image is the same size as it will be recorded on film, and erect but laterally reversed. In the twin-lens reflex camera, viewing and taking are by separate lenses; in the single-lens reflex camera, viewing and taking are by the same lens. Focusing and viewing are done using the unaided eye or a flip-up magnifier in the viewfinder hood. Unlike the SLR camera, the TLR camera suffers from parallax error, and a mask or indicator in the viewfinder may be coupled to the focusing mechanism, to compensate for this defect.

The mirror in the viewfinder system of a SLR camera necessitated refinements in camera design. The most necessary is an instant return mirror (with a damping system to minimize camera shake), otherwise the viewfinder is blanked out until the mirror returns to the viewing position when the film is advanced to the next frame. The mirror may have a lock-up facility for shake-free release in telephotography and photomicrography.

Most SLR cameras have a viewfinder that is some 5 per cent pessimistic in each direction, i.e. more appears in the picture than in the viewfinder. The reason usually quoted for this is to overcome differences in aperture size in transparency mounts. Others, however, do indicate the actual area included on the negative. A plain ground-glass screen indicates whether focusing is correct, but gives a rather dim image, with a rapid fall-off in illumination towards the corners. Evenness of illumination is improved if the screen is etched on the flat base of a plano-convex lens, or if an accessory Fresnel screen is used, as a field-brightening element.

Reflex cameras without autofocus usually have a supplementary focusing aid incorporated in the centre of the screen. This is a ‘passive’ device, i.e. it has no moving parts. It is usually in the form of a split-image rangefinder or a microprism array (described later). Professional cameras have interchangeable viewing screens for different applications. Many screens incorporate two or more focusing methods, called mixed screens. With all these screens, however, if the reflex mirror is too small there will be a progressive loss of illumination towards the top of the screen with increase in focal length of the camera lens. This cutoff (viewfinder vignetting) is not seen on the film image. The reflex mirror may be multi-coated to improve reflectance and give a brighter viewfinder image. In order to transmit a portion of the incident light to a photocell for exposure metering or to a photosensor array for autofocusing, the mirror may have a partially transmitting zone or a pattern of fine perforations, and a supplementary or ‘piggy-back’ mirror hinged to its reverse side.

Lateral reversal of the reflex viewfinder image is troublesome, especially in action photography. Fortunately, the addition of a pentaprism viewfinder gives an erect, laterally correct and magnified screen image for focusing (see Chapter 8); this is no doubt the main reason for the popularity of the SLR camera. These finders may be interchangeable with other types, and most have provision for eyepiece correction lenses or dioptric adjustment for users who would otherwise need spectacles for viewing. The viewfinder prism may incorporate a through-the-lens exposure metering system for measurements from the screen image.

Flash Synchronization

In the early days of flash photography it was customary to set the camera on a tripod, open the shutter on the ‘B’ setting, fire the flash (which was a tray of magnesium powder) and then close the shutter. As the manufacture of flashbulbs progressed, they became sufficiently reliable to be synchronized with the opening of the camera shutter. This made it possible for flash to be used with the shutter set to give an ‘instantaneous’ exposure, and the camera could be hand-held for flash exposures. At first, a separate synchronizer device was attached to the camera, but now flash contacts are incorporated in the shutter mechanism for both flashbulbs and electronic flash.

Between-Lens Shutters

Synchronization of between-lens and focal-plane shutters presents different problems. With a between-lens shutter, the aim is to arrange for the peak of the flash to coincide with the period over which the shutter blades are fully open. There is a short delay between the moment of release and when the blades first start to open (approximately 2 to 5 ms), and a further slight delay before the blades are fully open. With flashbulbs there is also a delay after firing while the igniter wire becomes heated, before combustion occurs and light is produced. By comparison electronic flash reaches full output virtually instantaneously.

To cope with both types of flash, a between-lens shutter is provided with two types of synchronization, called respectively X- and M-synchronization. With X-synchronization, electrical contact is made at the instant the shutter blades become fully open so this type can be used with electronic flash at all shutter speeds (Figure 9.16). It is also suitable for use with small flashbulbs at shutter speeds up to 1/30 s. Compact cameras with an integral electronic flash unit are X-synchronized only, as are hotshoe connections. With the now obsolescent M-synchronization the shutter blades are timed to be fully open approximately 17 ms after electrical contact is made. This was originally to allow large flashbulbs time to reach full luminous output when the shutter was fully open. This requires a delay mechanism in the shutter, and one is used similar to that used for the slower speeds in preset shutters. M-synchronization allows synchronization of type M (medium-output) flash-bulbs at all shutter speeds. It is not suitable for small flashbulbs (type MF) and certainly not for electronic flash, which would be over before the shutter even began to open, hence no image. This form of synchronization is now rare and only X-synchronization is usually provided. The exceptions are lenses with leaf shutters intended for large-format and some medium-format cameras. Some shutters manufactured in the early days of internal flash-synchronization have a lever marked ‘V-X-M’. The ‘V’ in this case is a delayed-action (‘self-timer’) setting, which is synchronized to electronic flash only.

Focal-Plane Shutters

Synchronization of focal-plane shutters presents a particular problem. Exposures with electronic flash can be made only at slower shutter speeds, where the shutter slit is the same size as the film gate, so that the whole film frame area can be exposed simultaneously by the flash. When large flashbulbs had to be synchronized with fast shutter speeds up to 1/1000 s, a special slow-burning flash-bulb (class FP), now obsolete, was available. This bulb gave a near-constant output for the whole transit time of the shutter blind. The modern electronic flash counterpart has a rapidly pulsed or ‘strobed’ output that permits the use of flash even at 1/12 000 s.

Unlike a ‘fully synchronized’ between-lens shutter, which has only one flash connection and a two-position switch to select X- or M-synchronization, some focal-plane shutters had one, two or even three flash connections in the form of PC (co-axial) sockets, hot shoes and special-fitting sockets. If only one, unmarked, outlet is fitted to the camera, such as a simple hotshoe or PC cable socket, the shutter is X-synchronized only. Some older cameras have a separate control linked to this sole outlet, able to select different forms of delay, but this too is now obsolete. A pair of outlets may be X and M, or perhaps X and FP. Use of the appropriate one of these with a suitable shutter speed automatically gives correct synchronization. The extension of exposure automation to the use of electronic flash resulted in most cameras now giving automatic setting of the electronically controlled shutter to the correct X-synchronization speed, when a suitable dedicated flashgun is attached. This reverts to an alternative setting for ambient light exposure while the flash is recharging.

In X-type synchronization, the electrical contact is made when the first shutter blind has fully uncovered the film gate, and before the second blind has begun to move. Depending on the design of shutter, X-synchronization may be possible up to a shutter speed of 1/300 s. Medium-format cameras have synchronization restricted to a maximum shutter speed of typically 1/30 to 1/125 s. Due to these comparatively long shutter-open times in relation to the brief duration of an electronic flash unit, the effect of the ambient light can be a problem, and an electronic flash exposure meter which also takes into account the shutter speed in use is helpful. A comparison of shutter operation and flash source characteristics is shown in Figure 9.17.

The alternative rear curtain synchronization is when the flash is triggered just before the second blind starts it travel. This is useful for longer shutter speeds to give a better visual effect when there is a mix of time exposure and flash exposure.

Flash Synchronization Connections

The interface between the electronic flash and camera shutter is by means of an electrical connection, usually in the form of a PC socket, hotshoe (and hotfoot) or special connection (such as by a pulse of infrared radiation to allow cordless operation in off-camera use).

PC (for Prontor-Compur) is a 3 mm co-axial socket and a frequent source of connection problems. Some form of locking or screw-in version is more reliable. Flash extension cables are also a source of problems. Multiple socket outlet adapters can be used for simultaneous firing of two or more flash units.

The hotshoe (Figure 9.18) is a simpler and more reliable single contact outlet, utilizing the original accessory shoe as used for viewfinders, range-finders and simple flashguns. The centre contact provides X-synchronization without trailing wires. Additional contacts in the hot shoe provide additional features with dedicated flashguns. There are also non-standard special shoe fittings in use. Flashguns may be provided with an interchangeable ‘hot foot’ to give dedicated features with different camera systems.

Red-Eye Effects

The original large size of flash units meant that the flashhead was positioned above and to one side of the camera. Many cameras now have integral flash units or use small units which clip on, both of which place the flash close to the camera lens. The result is near-axial lighting along the optical axis of the lens which can give ‘red-eye’ where the light illuminates the retina of the eye which is red with blood vessels. The result is undesirable and unattractive. The best remedy is to remove the flash unit from near the lens, e.g. on an extension cable or by use of a slave unit. Alternative bounce or indirect flash can be used, or a flash diffuser may help. There is a loss of effective output. For direct flash, multiple preflashes may help to reduce the pupil size, or an intense beam of light may be directed from the camera to the eye, or the subject may be able to look away from the camera or the ambient light increased. Animals show similar effects but the colour effect may be green.

Focusing Systems

Although lenses can be used satisfactorily as fixed-focus objectives, relying upon a combination of small apertures, depth of field and distant subjects to give adequate image sharpness, this simplification is not usually feasible with lenses of large aperture or long focal length, nor for close-up work. To ensure that the most important part of the subject is in sharp focus it is necessary to have some form of focusing system, as well as a visual indication of the state of focus. Current focusing systems use a variety of mechanical, optical and opto-electronic arrangements (Figure 9.19). Focusing is to satisfy the conditions of the lens conjugate equation where a change in subject distance (u) requires a corresponding change in image distance (v) with focal length (f).

Unit Focusing

The simplest method of focusing is by movement of the entire lens or optical unit (9.19a). This is called ‘unit focusing’. The lens elements are held in a fixed configuration. Movement is achieved in various ways. Technical cameras of the baseboard type employ a rack-and-pinion or friction device to move the lens on a panel for focusing by coupled range-finder or ground-glass screen. The monorail type is focused by moving either the lens or the back of the camera; this can be useful in applications such as copying, because rear focusing alters the focus only, whereas front focusing alters the size of the image as well as the focus.

Other cameras have the lens unit installed in a lens barrel or focusing mount. Rotation of a ring on the lens barrel moves the lens in an axial direction. A helical focusing mount causes the lens to rotate during focusing, whereas a rectilinear mount does not. Rotation is generally undesirable, as the various scales may not be visible at all times and optical attachments such as polarizing filters, which are sensitive to orientation, may need continuous adjustment. A special double helicoid arrangement is used to provide macro lenses with the extended movement necessary to allow continuous focus to magnifications of 0.5 or more. The focusing action may be coupled to a rangefinder or to an autofocus system, or it may be viewed on a screen. The focusing distance may also be set by a scale on the baseboard.

Front-Cell Focusing

In simple cameras fitted with between-lens shutters, focusing is possible by varying the focal length of the lens and not by varying the lens-to-film distance. This is achieved by mounting the front element in a cell with a coarse-pitch screw thread. Rotation of this cell alters the separation of the front element from the other groups (Figure 9.19b). A slight increase in this separation causes an appreciable decrease in focal length, giving a useful focusing range. Close focusing to less than one metre is not satisfactory because the lens aberrations introduced adversely affect performance. A distance scale is engraved on the movable cell. Many zoom lenses also use a (well-corrected) movable front group for focusing. Note that rotation of the front element is a nuisance with the use of polarizing filters and other direction-sensitive attachments such as graduated filters.

Close-Up Lenses

A positive supplementary lens can be fitted in front of almost any lens to give a fixed close-up focusing range (Figure 9.19c). If the subject is positioned at the front focus of the supplementary lens, the camera lens receives parallel light, so it gives a sharp image at its infinity focus setting. By varying the focus setting on the camera lens, a limited close-up range is given. The design of the supplementary lens is important, as the curvatures of its surfaces determine its effect upon the aberration correction of the prime lens. Usually a positive meniscus shape is used, with its convex side to the subject. This simple lens may be perfectly adequate when used at small apertures, but an achromatic cemented doublet design with anti-reflection coatings is preferable for improved performance. A +1 dioptre lens has focal length 1000 mm, and if of diameter 50 mm has an aperture of f/20, which allows a doublet design to give excellent correction. Typical examples are Zeiss Proxars and Leitz Elpro ranges, which may also be matched to particular prime lenses.

Close-up lenses are calibrated in dioptres, and positive powers of +1, +2 and +3 are commonly available. Not every manufacturer uses this coding. Powers of up to +10 are available, and the lenses can be used in tandem with the more powerful one closer to the prime lens. A variable power or ‘zoom’ closeup lens uses a variable separation between two positive menisci to give its dioptre range. Performance is generally poor. Fractional dioptre values of +0.25 and +0.5 are useful for telephoto, zoom and long focus lenses.

The effect of the supplementary lens is to reduce slightly the focal length of the combination. No exposure correction is necessary with supplementary lenses, and the aperture scale remains unchanged. The entrance pupil diameter is virtually unchanged by the addition of the supplementary lens. Single-lens reflex cameras pose no problem for close-up focusing and framing. Other types usually require an additional prismatic device to correct the viewfinder image for parallax errors due to the proximity of the subject. Twin lens reflex and rangefinder cameras are in this category.

Internal Focusing

A versatile method is internal focusing (Figure 9.19(d)), where the focusing control moves an internal element or group of elements along the optical axis, thereby altering focal length but not the external dimensions of the lens. The front of the lens does not rotate either. By this means an extensive focusing range is possible with only a small movement involving few mechanical parts. The lens is not extended physically, and the whole unit can be sealed against the ingress of dust and water. The focusing movement may be non-linear, and incorporate the correction for those lens aberrations which increase as the focused distance is decreased. The small masses and mechanical forces involved mean that internal focusing is particularly suitable for autofocus lenses. The drive motor can be located either in the camera body or in the lens housing. Internal focusing is used for a variety of lenses, including zoom, macro and super-telephoto types. Close-focusing is provided with retention of image quality.

Extension Tubes and Bellows

Many lenses can only be focused close enough to provide a magnification of about 0.1 or less. Most standard lenses have a minimum focusing distance of 0.3 to 1.0 metres. If the lens is removable, the use of extension tubes and bellows extensions between the lens and camera body provide the additional lens-to-image distance needed to give greater magnification. (Non-interchangeable lenses are limited to use of a supplementary close-up lens). An extension tube has fittings for attaching the lens to one end and the camera body to the other. Tubes of various lengths are available for use singly or in combination. Variable-length tubes are also available. For SLR cameras, automatic extension tubes have the necessary mechanical or electrical linkages to retain the operation of the iris diaphragm and TTL metering systems.

Together with the focusing movement on the lens mount, an extension tube gives a limited close-focusing range. At long extensions, a narrow-diameter tube may cause vignetting. For these reasons, an extension bellows is preferable as this permits a full and continuous focusing range, and allows lenses of long focal length to be used. Some lenses may be produced in a short-mount bellows-unit-only version. A 135 mm focal length lens for the 24 × 36 mm format with bellows may typically have a focusing range from infinity down to same-size reproduction. True macro lenses, i.e. lenses capable of full correction at magnifications greater than unity, are available specifically for bellows use.

The automatic diaphragm and other operations of a lens are retained with most designs of bellows by means of mechanical or electrical linkages, but where this is not so, a double cable release arrangement is needed to operate the iris diaphragm and shutter together (or the lens may need to be stopped down manually). Where magnification is greater than 1.0, the optical performance of a lens mounted in the usual way may be impaired, as corrections are normally computed for work at infinity, and macro settings reverse the usual proportions of conjugate distances (i.e. v >> u). A lens reversing ring, to mount the lens with its rear element facing the subject, avoids this problem. Extension tubes and bellows are most useful with SLR cameras, owing to the ease of focusing. A table is usually available listing increases in exposure for the various magnifications, but TTL metering avoids the necessity for this.

Focusing Scales

Lenses are usually focused without reference to the distance scales engraved on the focusing control when a rangefinder, focusing screen or autofocus system is provided. These scales are useful for reference when electronic flash is being used, so that the aperture may be set according to a flash guide number, and to check if the subject is within the operating distance range of an automatic flash exposure system, as described in Chapter 3. Also, the distance values can give an estimate of the depth of field by reference to the appropriate scales on the mount, or to tables. The focusing scale may also carry a separate index for infrared use, usually denoted by a red dot or letter ‘R’, and the distance value first set visually or by autofocus is then transferred manually to the IR index.

As the focal length of a lens determines the amount of extension necessary for focusing on a nearby object, the closest marked distance on a focusing scale varies with the particular lens in use. The small extension required with wide-angle, short-focus lenses means that many have provision for focusing down to a few centimetres. The standard lenses used with 24 × 36 mm format cameras commonly focus down to approximately 0.5 m, while macro lenses have provision for magnification of 0.5 to 1.0, and additional scales of exposure increase factors and reproduction ratios. Long-focus lenses may focus no nearer than 2 to 10 m, depending on their focal length, without recourse to extension tubes, bellows or closeup lens attachments. The long bellows extension of a technical camera and the possibility of increasing the extension by adding another section, makes for a very versatile focusing system. The usual triple-extension bellows allows a magnification of 2.0 with a standard lens, but may need replacement by a flexible bag-bellows for use with short-focus lenses, as the pleated type will not compress sufficiently to allow the lens to be focused on infinity.

Focusing mechanisms vary in their ease of use. Focusing rings, knobs and levers vary in size and width. The amount of friction, ease of gripping and direction of rotation for focusing differ between lenses, and this can cause confusion when several lenses are in use. A locking control is desirable to allow focus to be preset in several positions or to prevent accidental alteration during use (such as when copying). A zoom lens may have three raised rings on the lens barrel for the alteration of focus, aperture and focal lengths; careful design is needed to avoid confusion of function. Zoom lenses may use one touch operation: a control that rotates to change focus and slides to change focal length; systems using separate controls are more common. There may be a focus limiting control to restrict the focus range, to speed up autofocus and prevent ‘hunting’ for focus.

Ground-Glass Focusing Screen

The traditional ground-glass screen or focusing screen is a most adaptable and versatile focusing system. It has the advantages of giving a positive indication of sharp focus and allowing the depth of field to be estimated. No linkages between lens and viewfinder are necessary, apart from a mirror (if reflex focusing is used). The screen may be used to focus any lens or optical system, but focusing accuracy depends on screen image luminance, subject contrast and the visual acuity of the user. Supplementary aids such as a screen magnifier or loupe and a focusing hood or cloth are essential, especially when trying to focus systems of small effective aperture in poor light. The plain screen has evolved into a complex optical subsystem, incorporating passive focusing aids (see below) and a Fresnel lens to improve screen brightness. The screen can be a plate of fused optical fibres; alternatively, the glass may have a laser-etched finish to provide a detailed, contrasty, bright image. No single type of screen is suitable for all focusing tasks and many cameras have interchangeable focusing screens with different properties. Accurate visual focusing is aided by accurate adjustment of the dioptric value of the eyepiece magnifier or loupe to suit the vision of the user.

Coincidence-Type Rangefinder

Coincidence-type rangefinders usually employ two windows a short distance apart, through each of which an image of the subject is seen. The two images are viewed superimposed, one directly and the other after deviation by an optical system (Figure 9.20a). For a subject at infinity the beamsplitter M₁ and rotatable mirror M₂ are parallel, and the two images coincide. For a subject at a finite distance u the two images coincide only when mirror M₂ has been rotated through an angle x/2. The angle x is therefore a measure of u by the geometry of the system, and may be calibrated in terms of subject distance. By coupling the mirror rotation to the focusing mount of the lens in use, the lens is automatically in focus on the subject when the rangefinder images coincide. The accuracy of this coupled rangefinder system using triangulation is a function of subject distance u, the baselength b between the two mirrors and the angle x subtended at the subject by the base-length. Mechanical and optical limitations make this system of focusing unsuitable for lenses of more than about two-anda-half times the standard focal length for the film format, e.g. 135 mm for the 24 × 36 mm format. The method is unsurpassed in accuracy for the focusing of wide-angle lenses, particularly with a long-base rangefinder and in poor light conditions.

As the focused distance changes from infinity to about 1 metre, the required rotation of the mirror is only some 3 degrees, so high mechanical accuracy and manufacturing skill are required to produce a reliable rangefinder. Alternative systems have been devised. One of these employs a fixed mirror and beamsplitter, obtaining the necessary deviation by a lens element which slides across the light path between them (Figure 9.20b). The rangefinder images are usually incorporated into a bright-line frame viewfinder, one of the images being in contrasting colour for differentiation. Rangefinders add little to the bulk or weight of a camera.

Split-Image Rangefinder

A split-image rangefinder is a ‘passive’ focusing aid in that it has no moving parts, unlike the coincidence type rangefinder. It is a small device, consisting basically of two semi-circular glass prisms inserted in opposite senses in the plane of the focusing screen. As shown in Figure 9.21, any image that is not exactly in focus on the central area of the screen appears as two displaced halves. These move together to join up as the image is brought into focus, in a similar way to the two images of the field in a coincidence-type rangefinder. The aerial image is always bright and in focus, even in poor light conditions. Because focusing accuracy by the user depends on the ability of the eye to recognize the displacement of a line, rather than on the resolving power of the eye, which is of a lower order, the device is very sensitive, especially with wide-angle lenses at larger or moderate apertures. The inherent accuracy of this rangefinder depends on the diameter of the entrance pupil of the lens in use, so large apertures improve the performance. Unfortunately, the geometry of the system is such that for effective apertures of less than about f/5.6 the diameter of its exit pupil is very small, and the user’s eye needs to be very accurately located. Slight movements of the eye from this critical position cause one or other half of the split field to black out. Consequently, long-focus lenses or closeup photography result in a loss of function of the rangefinder facility and produce an irritating blemish in the viewfinder image. Since this obscuration is related to the prism angle, alternative prisms with refracting angles more suited to the lens aperture in use, or even of variable, stepped angle design, can retain useful rangefinder images even at small apertures.

The image-splitting arrangements may be vertical, horizontal, at 45 degrees, or use a more complex arrangement to cope with subjects without definite linear structures. An additional focusing aid is often provided such as an annular fine ground-glass ring around the rangefinder prisms. The split-image range-finder is popular with wearers of spectacles and others with refraction errors of vision, who may find it difficult to focus a screen image.

Microprism Grids and Screens

The principle of the split-image rangefinder is used in the microprism grid array, also located in the centre of the viewfinder focusing screen. A large number of small facets in the shape of triangular or square base pyramids are embossed into the focusing screen surface (Figure 9.22). An image from the camera lens that is not precisely in focus on the screen undergoes multiple refraction by these facets so that the image appears broken up into tiny fragmentary areas with a characteristic ‘shimmering’ effect. At correct focus this image snaps back into its correct appearance, giving a very positive indication of the point of focus. The pyramids are small enough (<0.1 mm diameter) to be below the resolving power of the eye. This system is easy to use and very popular. Again, no moving parts are used. The disadvantage of the microprism is the same as that of the split-image rangefinder, the microprism array blacking out at effective apertures less than about f/5.6. Interchangeable viewfinder screens that consist entirely of microprisms are available. These give a very bright image for focusing; they can be obtained with prisms with angles suitable for lenses of a particular focal length. Microprism arrays are generally used in conjunction with one or more of the other focusing aids; thus a central split-image rangefinder may be surrounded by annular rings of microprisms and ground glass, with the rest of the screen area occupied by a Fresnel lens.

Autofocus Systems

Autofocus Modes

For many purposes a lens that is focused visually, or even a fixed-focus lens, is adequate; the various optical aids described above simply improve accuracy of focusing. But visual focusing can be slow, inaccurate and tiring, especially if continuous adjustments are necessary. Some means of obtaining or retaining focus automatically is helpful, especially when using long-focus lenses and following subjects moving obliquely across the field of view in poor light; also for unattended cameras operated by an intervalometer. Various forms of autofocus system have been devised; these can operate in various modes. The autofocus system can be linked to the shutter release so this is blocked until focus is achieved, otherwise it can be overridden. The focus may then lock at this setting or it can search continuously for focus as the camera or subject move. An ‘active’ type of autofocus will scan a beam of infrared radiation across the scene, the reflection is measured and triangulation is used, similar to the coupled rangefinder. A ‘passive’ type uses no moving parts and a pair of images are compared by the output from a CCD array, similar to a split-image range-finder. The system can be used on demand, useful as a prefocus feature as the sensor area may be much smaller than the field of view of the lens; also, the composition may be such that an active system would give sharp focus on an area other than the main subject feature (for example, the background seen between the two heads, in a double portrait). A combination of active and passive systems may be combined to deal with a range of distances, the former being better for close distances, the latter for distant subjects.

A focus indication mode or assisted focusing mode is a form of electronic rangefinder being an analogue of an optical rangefinder or focusing screen. An LED display lights up in the viewfinder to indicate correct focus when the lens is focused manually; the direction of focusing movement required is indicated if the focus is incorrect. A predictive autofocus mode anticipates the position of a moving subject after the time delay due to the mirror rise time and will ‘drive’ the lens to this focus setting in compensation. An automatic depth-of-field mode is where the lens is focused automatically on the near and far points of the scene to be rendered in focus, the necessary intermediate focus is set and the automatic exposure determination system then chooses the appropriate aperture to give the correct depth of field, given an input of data from the lens as to its focal length even when using a zoom lens. Another type is a motion-detection mode, or focus-trap mode where the camera is autofocused at a suitable distance; when a moving subject enters this zone, the camera automatically fires the shutter. A focus maintenance mode retains sharp focus once this has been set visually; this is the system commonly used in ‘autofocus’ slide projectors.

Ideally, autofocus systems should be rapid-acting, should not ‘hunt’, should continue to function at low light levels, be economical with batteries, add little volume or weight to the camera, be reliable, and give accuracy comparable to or better than visual methods. Different systems are in use.

Active Ranging Systems

One autofocus method uses direct measurement of the subject distance. This ranging method using pulses of ultrasound was introduced by Polaroid for use in certain of their range of cameras for self-developing film. A piezo-electric ceramic vibrator (PECV) emits a ‘chirp’ of ultrasonic frequencies while the lens moves to focus from its near distance to infinity. The elapsed time for the return journey to the subject is proportional to subject distance. The return echo is used to stop this focusing action. Either a turret of supplementary lenses or a single elongated aspheric element may be rotated behind the main camera lens to vary the focus. This system operates even in the dark, but cannot penetrate glass.

Other ‘active’ opto-electronic systems involve some means of scanning across the subject area (see Figure 9.23). An electronic distance measurement (EDM) system uses an infrared-emitting diode moving behind an aspheric projection lens to scan a narrow beam across the scene. Reflection from a small well-defined subject area is detected by a photodiode giving a maximum response, and the scan action is halted, as is the synchronous focusing movement of the camera lens. The original Honeywell Visitronic module used a subject-scanning system, where a rotating mirror synchronized to the motorized focusing movement imaged a zone of the subject onto a photodetector array; this response is compared to the static response of the subject area as seen by a fixed mirror and second detector array. The subsequent correlation signal is a maximum when both mirrors image the same zone. This signal then locks the focusing travel. This is the electronic analogue of a coupled rangefinder. The system works best at high light levels and is dependent on specialized signal-processing circuitry. Another system (due to Canon) uses two fixed mirrors to image zones of the scene cross a linear CCD array of some 240 elements. The focused image area is compared with adjacent image areas along the array, and maximum correlation gives a ‘base’ or distance between the two correlated regions which relates to the subject distance and an angular subtense. This continuous self-scanning triangulation system can be adapted to operate through the lenses of video cameras.

Systems using infrared beams can operate in darkness and through glass, but can be fooled by unusual reflectances of the radiation from various materials such as dark cloth.

Phase Detection Focusing

This passive system uses one or more linear CCD arrays and no moving parts. The CCD array is located in the mirror chamber (Figure 9.24) beyond an equivalent focal plane and behind a pair of small lenses which act in a similar manner to a prismatic split-image rangefinder, as in Figure 9.25. Divergent pencils of rays beyond the correct focus position are refracted to refocus upon the array, and the separation (or ‘phase’) of these focus positions relative to a reference signal is a measure of the focus condition of the camera lens. The phase information operates a motorized focus control (Figure 9.26) in the lens, so the motor is suited to the torque requirements of individual lenses. Very rapid focusing is possible, even in continuous-focus mode. The system will even focus in the dark, using the emission of infrared radiation from a suitably filtered source in a dedicated flashgun. The autofocus zone is indicated in the viewfinder of a camera and this can be located on the important detail of the subject to be sharply in focus. The focused distance data may be used in exposure calculations or in the operation of dedicated flash systems. Accuracy of operation is dependent upon using a lens aperture of f/5.6 or larger and also a reasonable light level. Low light performance is given by an EV scale of values, similar to that used for exposure metering systems. A dedicated flashgun fitted to the camera may emit a small preflash with a striped pattern to fall on the subject and assist autofocus in dim light. Alternatively, a continuous beam of deep red light or even infrared may be used.

One operational disadvantage is that, as the optical design of the autofocus system involves a beamsplitter mirror, the operation of which is affected by linearly polarized light, the system may not work correctly if a linear polarizing filter is used over the camera lens. A circular polarizing filter is instead required.

To deal effectively with a variety of subjects which may contain little correctly orientated detail or even none on which the autofocus system may ‘lock’, and to avoid the incorrect focus that may be set if the subject is not central in the frame, a variety of designs of autofocus arrays are in use in different cameras (Figure 9.27). One or more of the arrays in the field of view may be selected as suitable, sometimes by looking at the zone when an ‘interactive’ viewfinder will activate the selection and light a suitable indicator.

Exposure Metering Systems

Exposure

The topics of camera and flash exposure determination are dealt with fully in Chapter 19. An exposure meter with a calibration constant (K) measures the luminance (L) of the subject by reflected light, and given a film speed (S) the exposure equation t = KN²/LS is solved by an integral calculator to display the various combinations of shutter speed (t) and f-number (N) required. The value of film speed may be automatically input by a DX coding system. The values of N and t may be selected automatically by a selected program mode. The incorporation of an exposure metering system into a camera body is more than simply a question of convenience for the user: it is a crucial element in the design. A variety of types are used, each with its own advantages and limitations. When colour materials are used, accurate exposure is particularly important. Separate exposure meters are still used, but the transfer of indicated values of shutter speed and aperture from meter to camera is time-consuming. An integral meter coupled to the camera controls gives more rapid operation; and there are numerous possibilities of exposure automation. The evolution of the built-in meter has gone through several stages, each of increasing complexity; but before considering these it is useful to review the properties of the various types of photocell in use in light-metering systems.

Photocells

Selenium Cells

Light-sensitive selenium (Se) is the active element in a photovoltaic cell. Exposure to light generates an electric potential across the cell. A sensitive galvanometer in the circuit gives a deflection proportional to the incident light incident, and the necessary camera exposure is derived from the light value (LV) reading, usually via a calculator disc mounted on the meter. Sensitivity is limited, depending on the area of the cell exposed to light. A baffle limits the acceptance angle of the cell to approximately that of a camera with standard lens. Selenium cells are now rarely used as exposure automation is difficult other than by ‘trap-needle’ methods.

Cadmium Sulphide Cells

The action of light on a cadmium sulphide (CdS) photocell lowers its electrical resistance, and increases the current from a d. c. power source connected across the cell. A sensitive galvanometer in the circuit is calibrated accordingly. A small, long-life power cell of constant voltage is connected in series in the meter circuit. The CdS device is called a photo-resistor, and is very small; but has a much greater sensitivity than a selenium cell. Its spectral response is adjusted to approximately match that of a selenium cell. The disadvantages of the CdS cell are its temperature-dependence, its ‘memory’ and slow speed of response. The ambient temperature can affect the response of the cell and its calibration. Response depends on the previous history of illumination so that a reading taken in a low light level after exposure of the cell to a much higher light level, tends to be an exaggerated response due to a ‘memory’ of the previous light level. A significant time is required before response is back to normal. Finally, the speed of response in dim light is slow, the meter needle usually taking several seconds to reach its final reading. The CdS cell has largely been replaced by the silicon photodiode.

Silicon Photodiode

Light incident upon a solid-state device called a silicon (Si) photodiode (SPD) generates a very small current. Like the selenium cell this device is photo-voltaic. But its output is too low to be used in the same way, and in cameras an amplifier is necessary to produce a useful output. An operational amplifier acts as a current-to-voltage converter, and with a suitable feedback resistance gives a high output voltage, linearly proportional to the incident light. The response is good even at very low light levels, and the linearity of response is maintained over a wide range of illuminance levels. The response time of an SPD is very short, of the order of microseconds, a useful property for switching functions. The cell area can be very small while retaining adequate sensitivity (a useful property for incorporation into a camera body). Suitable solid-state amplifier modules provide a photocell device with properties superior to those of CdS cells.

Like the CdS cell, and in contrast to the Se cell, the spectral sensitivity of the SPD cell extends from about 300 nm to about 1200 nm, with a peak around 900 nm. For photometric use in a camera or light meter, filtration to remove much of this natural IR sensitivity is necessary to give a suitable response. Such filtered varieties for use in the visible spectrum are often termed ‘silicon blue’ cells. Comparative electrical circuits for the operation of Se, CdS and SPD photocells are shown in Figure 9.28.

Integral Metering Systems

Many cameras have integral (built-in) light metering systems. The three types of photocell may be used with the cells external to the camera body, positioned in a variety of locations and ways. Because of their large size, selenium cells were generally located in the front plate of the camera or in a ring round the lens (now rare). The acceptance angle usually matches that of a non-interchangeable standard lens. The smaller CdS or SPD cells have greater choice of location, but are generally positioned behind a small lens and aperture in the front plate or in the lens surround. The acceptance angle is usually smaller than that of the standard lens. Provision must be made for a battery and an on–off switch for the meter cell. Often this is by ‘first pressure’ on the shutter release.

Other cameras with integral meters have the photocells positioned within the camera body so that light measurement is made through the camera lens itself, and the lens can be interchangeable.

Through-The-Lens Measurement

The small size and high sensitivity of CdS and SPD photosensors enable the taking of reflected-light measurements from the subject through the camera lens (TTL) by a photocell installed in the camera body. In theory, this should automatically compensate for the transmission losses of the lens in use, for any lens extension used, for close focusing and for the use of filters, perhaps with limitations in the last case. Measurement may be from all or only part of the subject covered by the lens in use, depending on the metering mode selected. Such a system is easily incorporated in a SLR camera and the TTL metering system is either fixed in the camera or added by means of an alternative viewfinder housing incorporating the metering system.

Many different arrangements are possible for TTL metering systems and current camera designs use most of these.

Position of the Meter Cell

The photocell(s) can be in various shapes or sizes without much effect on sensitivity; this allows great flexibility in the choice of location within the camera body (Figure 9.29). Ideally, the cell should be located close to the film plane, but the presence of a focal-plane shutter hinders this. A cell on a hinged or retractable arm which locates it just in front of the shutter blinds has been used, but measurements are more commonly made with the cell in an equivalent focal plane. This is a plane located at the same distance as the film plane from the exit pupil of the camera lens. One such plane is that of the ground-glass screen used for focusing, and this can be monitored by photocells in the viewfinder housing. A beamsplitter system can divert light from part of the screen to a cell located outside the screen area. The area used for measurement purposes is clearly marked on the screen.

Another equivalent focal plane is located in the base of the dark chamber or mirror housing of the camera body. A partially transparent or perforated area of the main reflex mirror transmits light via a small subsidiary mirror, termed a piggy-back mirror, down into the well of the dark chamber where a complex photocell arrangement is located. The cell may be a segmented type or have a supplementary aspheric lens which can be selected to give a different form of metering pattern. The autofocus system may be located here also.

A convenient but seldom-used location is to place the photocell(s) behind all or parts of the reflex mirror, with access to light by an arrangement of slits in the mirror or by making the mirror partially transparent. An additional cell reading in opposition may be needed to cancel out the error effects of light reaching the cells from the viewfinder system. Such an arrangement is unsatisfactory with interchangeable lenses if there is a change in position of the lens exit pupil. Different lenses may have exit pupils that are at different distances from the mirror along the optical axis. The measurement area of the reflex mirror then intersects the cone of light from the lens exit pupil in different positions, and samples different cross-sectional areas of the cone.

A common location is to put photocells in the housing of the pentaprism to measure the luminance of the image on the focusing screen, remembering that a change of focusing screen type may need a correction applied to the metering system, owing to changes in screen image luminance. Such arrangements also allow an interchangeable pentaprism housing with TTL metering to be offered as part of a camera system. This alternative is favoured for medium-format SLR cameras. The cell or cells are located either behind a pentaprism face or around the eyepiece lens. A single cell monitoring the whole of the screen area may be used, but generally two cells are employed to give a weighted reading to favour the central zone of the screen. This can also offset uneven illumination of the screen caused by lenses of different focal lengths. A choice of weighted or small-area readings may be possible. Use of a segmented photocell allows different modes of use by measurement and comparison of scene luminance in different zones, sometimes termed matrix metering. Another ingenious solution is to position a fast-reacting SPD to respond to light reflected from the film surface itself, i.e. to the image luminance, and to terminate the exposure duration by controlling the shutter operation or by terminating an electronic flash emission. This is referred to as off-the-film (OTF) metering. Such an arrangement may also monitor changes in the subject luminance during the exposure duration, when the mirror is raised and when other metering systems would not be operating. It has a sufficiently rapid response to monitor the output of a dedicated electronic flashgun with the necessary thyristor control circuitry. To allow for the incomplete uncovering of the film gate by the slit in the blinds of the focal plane shutter when giving short exposure durations, the shutter blinds may be printed with randomized arrays of light and dark patches to simulate the reflectance of the film surface and provide a form of weighted reading if required. Again, a set of segmented photocells can provide a form of matrix metering with electronic flash.

Type of Reading

A TTL metering system is based on measurement of the light reflected from the subject as transmitted by the optical system in use. The meter cell does not, however, always measure all the light from the subject area covered by the lens. Several different systems are in use (Figure 9.30). A ‘fully integrated’ reading is a measure of all the light from the subject area, and the exposure indicated or given is liable to error caused by the tonal distribution and luminance range of the subject matter. An alternative is a ‘spot’ reading, where a very small area of the subject is measured; but a mid-tone grey region must be selected. There may be the choice of using a shadow or highlight spot reading and even facilities for taking several such readings from chosen parts of the scene, when the metering system will give an average reading based on these.

The ‘small area’ type of reading is an integrated measurement of an area of the subject too large to be considered a spot reading, but it is not a fully integrated one. This choice can give a useful compromise. The ‘weighted’ reading is also a compromise where the whole image area is measured, but the central portion of the subject as viewed contributes most towards the result given. Various weighting patterns are in use, favouring different regions of the scene, for example to compensate for a bright sky in the upper region.

A segmented photocell arrangement, which takes several discrete or overlapping measurements simultaneously from different zones of the scene, permits rapid changeover between different metering patterns or modes, and in combination with a memory programmed with a large range of optimum exposures for various scene luminance ranges and luminance distributions, the metering system may give an optimum exposure for the circumstances. The distance of the subject is also factored into the calculations (see Chapter 19). The use of fuzzy logic blurs the otherwise sharp delineation between scene categories and reduces the possibility of a sudden change in exposure with a small change in composition.

In circumstances where the metering system in use may give a wrong result due to non-typical tone distribution or luminance ratio, the use of an exposure memory lock control can be useful. The camera is directed at a chosen part of the scene (or at an alternative subject) and the indicated exposure settings are memorized. The camera is then pointed at the previous scene as composed and an exposure made with the predetermined values. The memory may be self-cancelling, or the data locked in until erased.

The multiplicity of methods of readings, combined with the different locations possible for the meter cell, mean that there are a number of choices of TTL-metering in cameras.

Sensitivity

The metering sensitivity of a TTL system is generally similar to a good selenium-cell meter for hand-held use. This is due to the loss of light by the optical and viewing system of the camera. A lens of large maximum aperture improves sensitivity, unless a stop-down measurement system is used. A SPD with amplification circuitry increases sensitivity. To compare sensitivities, it is usual to quote the sensitivity as the minimum EV number that the system can determine as an exposure when ISO 100 film is in use. For example, a sensitivity of EV 1 is typical of an SLR TTL system. A hand-held light meter, by comparison, can have an equivalent sensitivity of –4 to –6 EV, factors respectively of 32 and 128 times the TTL value.

Operation of the Metering System

An integral metering system is of great practical assistance, but its operation varies. Numerous steps are involved. First, the battery power must be switched on, often by first pressure on the shutter release button. This may then initiate a battery check function and a timing circuit to limit battery use and conserve life. Then the subject must be sharply focused and the appropriate area selected for measurement if applicable. In semi-automatic exposure modes either the shutter speed or lens aperture is selected first, and then the other variable set as indicated by the meter. This may be done by ‘match-needle’ operation, i.e. by altering the appropriate control until a moving needle in the viewfinder readout is brought into coincidence with a fixed needle or cursor. Alternatively, an LED display may indicate the necessary setting by a flashing symbol or by a simple plus, zero and minus signs which light up as the settings are changed. Stop-down metering, where the operation is done at the chosen aperture, is now rare except for instances such as close-up photography, or for use with lenses lacking a fully automatic iris diaphragm. In fully automatic cameras there is usually a choice of one or several alternative metering modes, and the user may simply have to select one setting of aperture or shutter speed or just a letter such as S, A or P on a control dial or LCD display. The autofocus system may operate at the same time.

Automatic Exposure Modes

In the aperture-priority mode the user selects a lens aperture best suited to purposes such as depth of field or optimum lens performance. Sometimes the effective aperture may not be known of the optical system in use, such as a microscope. The metering system then gives the appropriate exposure duration by control of the shutter speed. The exposure could be of many seconds duration, when reciprocity-law failure might be anticipated, but the duration also depends on metering sensitivity. An indication is given when the exposure duration necessary is out of the shutter speed range. The use of OTF metering in ‘real time’ compensates for fluctuations in scene luminance during the exposure.

In the shutter-priority mode the user selects the shutter speed most appropriate to avoid camera shake or to stop a moving subject or to allow the use of electronic flash fill-in lighting. The metering system then determines the appropriate aperture. The lens usually has a separate lockable ‘A’ setting on the aperture control ring for this mode. Various mechanisms set the aperture to the required fractional setting, possibly to 0.1 of a stop, using motorized operation within the lens.

In the programmed mode the camera metering system selects a suitable combination of shutter speed and lens aperture from a program of combinations related to the subject luminance. More details are given in Chapter 19. The program may be selectable from several on offer, set by the user, and may also receive an input from the lens in use to determine the appropriate shutter speed. Programs that favour small apertures for maximum depth of field are also used. The modes may be given labels such as ‘depth’ or ‘action’ or ‘portrait’, where, respectively, the mode selects a small aperture for depth of field, a high shutter speed to freeze a moving subject, or a large aperture to blur the background.

When using TTL flash metering, the usual arrangement is for the user to select an aperture; the flash is quenched appropriately, giving a measured exposure duration. But programmed flash is also possible where the subject distance, as focused first, will determine the lens aperture for a given flash output. Other alternatives are possible.

Battery Power

Modern cameras, with few exceptions, are totally dependent upon battery power for all or most of their functions. Exceptions are cameras with a mechanical shutter and lacking an exposure meter or use a selenium cell meter. Battery power is used for metering, autofocus, microprocessor operation, electronic shutter control, self-timer, viewfinder displays, and motors to advance the film, rewind it and focus the lens. Accessories such as power winders, flash-guns, data backs and remote controls all use batteries. Without functioning batteries the equipment is inoperative. An exception is a ‘mechanical’ setting of a shutter providing a single speed suitable for flash synchronization, given a failure of the battery operating the shutter timing circuitry.

For camera equipment only low direct current (DC) voltages are required, seldom more than 6 volts. Small batteries are favoured given space limitations, but have reduced capacities, especially given the number of functions to be powered. Batteries should be inserted correctly regarding polarities. A small vent hole in the battery compartment cover allows the escape of gases produced in operation. Batteries may be in a removable holder or the compartment form part of a shaped grip on the camera body. Battery performance is related to temperature and a cold weather adapter kit may be available to allow the battery pack to be kept in a warm pocket and a long cable terminates in an adapter to go into the camera compartment. A battery condition test and indication is vital. Alternative possibilities include a button and LED signal, or a warning symbol in an LCD display of status. To conserve battery power a timing circuit may switch off the camera after a short time, varying from 30 seconds to several minutes. This switch-off can occur at the most inconvenient moment, when a picture is about to be taken. Slight pressure on the shutter release will keep power on. Other means of saving battery power include having mechanical latches on camera shutters for long exposure times using the ‘B’ setting. Electronic flashguns use very effective thyristor-controlled energy-saving circuitry to discharge the capacitor only as much as necessary, also the unit may switch off if not used for a few minutes. Battery drain depends on circumstances, type of circuit and type of battery. Some metering or shutter control circuits are largely independent of the state of the battery, others such as SPD circuits require a stabilized voltage, so batteries are favoured that have a near constant output during their operation and life. Heavy duty applications such as motor winders and flashguns need moderate voltage and short bursts of heavy current, supplied by clusters of alkaline-manganese or nickel-cadmium cells. Medium format cameras with integral motor drives also have heavy-duty needs.

A battery uses cells in series and each electro-chemical cell uses dissimilar electrodes and a suitable conducting electrolyte, which can be liquid, gaseous, viscous or near solid. Leakage of electrolyte can be disastrous and damage equipment severely, so batteries should be removed when equipment is not needed for a time. Chemical reactions cause a gradual loss of voltage, from an initial value which is some 1.2 to 2 volts per cell. Shelf life varies for unused batteries, but can be five years or more in the case of lithium types. A battery tester may be incorporated to check voltage before use. Batteries should be changed in sets, not just one discarded when found to be discharged. Batteries are either of the throwaway or rechargeable type. The first is disposed of when exhausted and it is very dangerous to attempt to recharge it as the gases formed may cause it to explode. The rechargeable cell uses a trickle DC current of low voltage to reform the chemical cell by inducing reverse reactions. With such cells, too rapid a discharge or charge must be avoided as internal damage and heating may occur. Types of rapid recharging batteries are available such as lithium ion and nickel metal hydride types. The popular nickel– cadmium (NiCd) type may be recharged several hundreds of times in a normal lifetime of use, but does not last indefinitely. The lower internal impedance of this type can give shorter recycling times in flashguns. Batteries should be inspected for corrosion before insertion and the terminals wiped with a cloth to remove any oxide film and improve contact.

A number of battery types are in use. The carbon– zinc cell, the oldest type, is cheap, has a short life and cannot deliver large amounts of current. It is best used only in an emergency if other varieties are unobtainable. Zinc chloride types give improved performance. The alkaline cell, or manganese-alkaline type is similar to the carbon–zinc type but with a different electrolyte and near-leakproof construction. It has about ten times the life and a wide operating temperature. The mercury button cell is no longer available for safety reasons and has been largely replaced by the silver-oxide cell which has a very stable output but costly so frequently substituted by alkaline or lithium cells. The lithium cell uses lithium instead of zinc for its positive terminal. Combined with manganese dioxide a long shelf life is given and a useful life in a camera. Low temperature operation is possible to −20 ° C. The smallest voltage available is 3 volts so it can only replace two silver oxide types used in series. The nickel–cadmium cell delivers 1.25 V that is usually adequate unless 1.5 V is essential. Newer types include the lithium ion and nickel metal hydride types, both of which are rechargeable and retain no memory of previous use.

Data Imprinting

It is sometimes useful to record data associated with the image for later purposes, such as unique identification, archiving and retrieval. The data may be added during the photographic or recording process. A variety of methods and systems are used. Note that the edges of photographic material already carry a large amount of latent data imprinted during manufacture, such as codings for film type, batch, frame numbers, footage etc. These can be used for filing and editing purposes, act as a simple quality control system for processing, and also allow the image to be correctly orientated. Sheet film materials carry a complex notching system for identification. Edge-printed bar codes and binary codes as perforations in the film tongue are used for automated printing purposes. The film material may also use a transparent magnetic coating to carry information in various forms. Imprinted data may possibly only be machine readable or retrieved electronically.

Data may be either imprinted during exposure, or during wind-on, or after processing, and can be located within the image or in the rebates beside or between frames, or even on the blank frames incurred during film loading. Many cameras can be fitted with accessory data backs or command backs to provide an additional range of functions. The data can be analogue (such as a watch face or dials) or in dot matrix form using LEDs and relayed optically through the film base to imprint during exposure. The exposure for the data is frequency modulated to relate to the film speed used. Exposure is synchronized to the camera shutter, sometimes via a flash socket. Options include the date in various formats, time likewise, frame number and magnification. Imprinting data within the frame is inconvenient and can obscure vital information, it is better to print the data on rebates. Command backs supply a wide range of data such as shutter speed, aperture, program mode, frame number, focal length, date, time, use of flash etc. An alternative is to have the specified data for each frame encoded and stored in RAM memory in the camera or command back, to be later interrogated and downloaded into a PC for printing. Exposure data for several rolls of film can be stored in this way. One system imprints all exposure data and frame information upon rewinding the film, using the first two frames, which have been deliberately left blank for the purpose.

The Advanced Photographic System (APS) first introduced in 1996 has a variety of data recording capabilities as it features a full area transparent magnetic coating, capable of encoding some 2–8 kbytes of information per frame. This information includes that specific to user and processing needs, such as format required and simulated focal length, but potentially, frame-related sound recording and control data as well. The full film length carrying images and magnetic data is retained encapsulated in its protective cassette after processing. Comprehensive data can be printed on the front or reverse of machine prints from the information exchange (IX) system used. A significant amount of exposure and other information can be printed on the reverse of a print.

A digital camera can record detailed information for each exposure, available when downloaded to a computer. Interfacing is possible with a mobile Global Positioning System (GPS) unit to provide the exact location of the viewpoint of an image by imprinting the latitude, longitude and elevation of the camera at the moment of exposure. This feature is useful where location is important in an application.

Bibliography

BS ISO 516:1999. Photography – Camera Shutters – Timing. British Standards Institute, London.

Crompton, T. (1995) Battery Reference Book, 2nd edn. Newnes, Oxford.

Goldberg, N. (1992) Camera Technology. Academic Press, San Diego, CA.

Ray, S. (1983) Camera Systems. Focal Press, London.

Ray, S. (1994) Applied Photographic Optics, 2nd edn. Focal Press, Oxford.

Ray, S. (1999) Scientific Photography and Applied Imaging. Focal Press, Oxford.