As we saw earlier (Page 18), the lens system contains an adjustable diaphragm or iris formed from a series of flat interleaving metal blades. Turning an iris control ring at the near end of the lens barrel adjusts the size of this hole – the lens working aperture or f-stop.
The ring has a graduated scale with a series of standard f-stop settings. These relate to the proportion of the light passing through the lens system, and falling onto the pickup sensor (the CCD). You select a particular f-stop setting, by aligning it with an indicator mark.
The larger the maximum size of the lens aperture, the more light it will pass. When shooting under lower light levels, a camera using a fast lens (with a maximum aperture of e.g. f/1.5), will produce better pictures than one with a smaller maximum aperture of e.g. f/5.6 (a slow lens).
There are several operational methods of adjusting the lens aperture:
• Iris control ring – Here you turn the iris control ring with the fingers of your left hand; the method widely used on portable video cameras.
• Remote control – Using this method, the f-stop is set to suit prevailing lighting conditions before shooting. Then as shots change, an operator at a remote CCU or vision control console (base station) varies the lens aperture slightly, to compensate for any unevenness in the lighting, or to enhance the artistic quality of the picture.
Studios are lit to a standard light intensity, and the equipment adjusted to suit this during a pre-rehearsal camera check (camera set-up; line-up).
• Auto-iris – An increasing number of cameras are provided with the option of an auto-iris, which self-adjusts the lens aperture to suit the scene brightness. We shall look at this facility more closely later, when considering automatic lens controls (Page 65).
Adjusting the lens aperture has two important effects simultaneously:
1. It controls the brightness of the lens image reaching the CCD from the lens. So it affects the picture’s exposure – i.e. the reproduced tonal values.
2. It will affect the depth zone within the scene (depth of field), in which subjects appear sharply focused.
In practical terms, as you will see, this means that you —
• Either adjust the lens aperture to expose the picture under prevailing lighting, and accept whatever depth of field results,
• Or select the f-stop that will provide the depth of field you require, and adjust the light intensity to give a correctly exposed picture (by adding illumination, or filtering the lens as appropriate).
Adjustment
The lens aperture is adjusted by turning a ring on the lens barrel. The f-stop number opposite the marker shows the aperture selected. As the lens is stopped (irised) down, its aperture becomes smaller. (Light-transmission is reduced and depth of field increases.)
F-stops and transmission numbers
Lens apertures are often calibrated in a standard series of calculated markings such as: f/1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22. Transmission numbers indicate the amount of light passing through a lens system at various apertures, and can be taken as equivalent to corresponding f-stops.
Light transmission
The effect on the amount of light passing through the lens system, on changing to a new aperture, equals:
Thus from f/4 to f/8 the amount of light received by the camera tube is reduced by 42/82 = ¼.
Light change
Opening a full stop increases the lens aperture to admit twice the original light e.g. f/8 to f/5.6. Opening half a stop increases the lens aperture to admit half as much light again as originally, e.g. f/8 to f/6.3.