As you learn to use your EOS R5 or R6 creatively, you’re going to find that the right settings—as determined by the camera’s exposure meter and intelligence—need to be adjusted to account for your creative decisions or to fine-tune the image for special situations.
For example, when you shoot with the main light source behind the subject, you end up with backlighting, which results in an overexposed background and/or an underexposed subject. The camera recognizes backlit situations nicely, and can properly base exposure on the main subject, producing a decent photo. Features like Highlight Tone Priority and the Auto Lighting Optimizer can fine-tune exposure to preserve detail in the highlights and shadows.
But what if you want to underexpose the subject, to produce a silhouette effect? Or, perhaps, you might want to use an external electronic flash to fill in the shadows on your subject. The more you know about how to use your camera, the more you’ll run into situations where you want to creatively tweak the exposure to provide a different look than you’d get with a straight shot.
This chapter shows you the fundamentals of exposure, so you’ll be better equipped to override the default settings when you want to or need to. After all, correct exposure is one of the foundations of good photography, along with accurate focus and sharpness, appropriate color balance, freedom from unwanted noise and excessive contrast, as well as pleasing composition.
You have a great deal of control over all of these, although composition is entirely up to you. You must still frame the photograph to create an interesting arrangement of subject matter, but all the other parameters are basic functions of the camera. You can let your camera set them for you automatically, you can fine-tune how it applies its automatic settings, or you can make them yourself, manually. The amount of control you have over exposure, sensitivity (ISO settings), color balance, focus, and image parameters like sharpness and contrast make the camera a versatile tool for creating images.
In the next few pages, I’m going to give you a grounding in one of those foundations, and explain the basics of exposure, either as an introduction or as a refresher course, depending on your current level of expertise. When you finish this chapter, you’ll understand most of what you need to know to take well-exposed photographs creatively in a broad range of situations with the camera.
This section explains the fundamental concepts that go into creating an exposure. If you already know about the role of f/stops, shutter speeds, and sensor sensitivity in determining an exposure, you might want to skip to the next section.
In the most basic sense, exposure is all about light. Exposure can make or break your photo. Correct exposure brings out the detail in the areas you want to picture, providing the range of tones and colors you need to create the desired image. Poor exposure can cloak important details in shadow or wash them out in glare-filled featureless expanses of white. However, getting the perfect exposure requires some intelligence—either that built into the camera or the smarts in your head—because digital sensors can’t capture all the tones we can see. If the range of tones in an image is extensive, embracing both inky black shadows and bright highlights, we often must settle for an exposure that renders most of those tones—but not all—in a way that best suits the photo we want to produce.
You’re probably aware of the traditional “exposure triangle” of aperture (quantity of light and light passed by the lens), shutter speed (the amount of time the shutter is open), and the ISO sensitivity of the sensor—all working proportionately and reciprocally to produce an exposure. The trio is itself affected by the amount of illumination that is available. So, if you double the amount of light, increase the aperture by one stop, make the shutter speed twice as long, or boost the ISO setting 2X, you’ll get twice as much exposure. Similarly, you can increase any of these factors while decreasing one of the others by a similar amount to keep the same exposure.
Working with any of the three controls involves trade-offs. Larger f/stops provide less depth-of-field, while smaller f/stops increase depth-of-field (and potentially at the same time can decrease sharpness through a phenomenon called diffraction). Shorter shutter speeds do a better job of reducing the effects of camera/subject motion, while longer shutter speeds make that motion blur more likely. Higher ISO settings increase the amount of visual noise and artifacts in your image, while lower ISO settings reduce the effects of noise. (See Figure 4.1.)
Exposure determines the look, feel, and tone of an image, in more ways than one. Incorrect exposure can impair even the best-composed image by cloaking important tones in darkness, or by washing them out so they become featureless to the eye. On the other hand, correct exposure brings out the detail in the areas you want to picture and provides the range of tones and colors you need to create the desired image. However, getting the perfect exposure can be tricky because digital sensors can’t capture all the tones we can see. If the range of tones in an image is extensive, embracing both inky black shadows and bright highlights, the sensor may not be able to capture them all. Sometimes, we must settle for an exposure that renders most of those tones—but not all—in a way that best suits the photo we want to produce. You’ll often need to make choices about which details are important, and which are not, so that you can grab the tones that truly matter in your image. That’s part of the creativity you bring to bear in realizing your photographic vision.
For example, look at two bracketed exposures presented at top in Figure 4.2. For the image at upper left, the darker areas and shadows are well exposed, but the brightest highlights—the ceiling fixtures and the scene outside the windows—are seriously overexposed. The version on the upper right, taken an instant later with the tripod-mounted camera, shows detail in the highlights, but most of the country store’s interior is cloaked in darkness. The camera’s sensor simply can’t capture detail in both dark areas and bright areas in a single shot.
With digital camera sensors, it’s tricky to capture detail in both highlights and shadows in a single image, because the number of tones, the dynamic range of the sensor, is limited. The solution, in this case, was to resort to a technique called High Dynamic Range (HDR) photography, in which the two exposures from Figure 4.2 were combined in an image editor such as Photoshop, or a specialized HDR tool like Photomatix and Aurora HDR (both about $100 from www.hdrsoft.com and www.skylum.com, respectively). The resulting shot is shown at bottom in Figure 4.2. I’ll explain more about HDR photography later in this chapter. For now, though, I’m going to concentrate on showing you how to get the best exposures possible without resorting to such tools, using only the features of your camera.
To understand exposure, you need to understand the six aspects of light that combine to produce an image. Start with a light source—the sun, an interior lamp, or the glow from a campfire—and trace its path to your camera, through the lens, and finally to the sensor that captures the illumination. Here’s a brief review of the things within our control that affect exposure.
These factors—the quantity of light produced by the light source, the amount reflected or transmitted toward the camera, the light passed by the lens, the amount of time the shutter is open, and the sensitivity of the sensor—all work proportionately and reciprocally to produce an exposure. That is, if you double the amount of light that’s available, increase the aperture by one stop, make the shutter speed twice as long, or boost the ISO setting 2X, you’ll get twice as much exposure. Similarly, you can increase any of these factors while decreasing one of the others by a similar amount to keep the same exposure.
If you’re really new to more advanced cameras (and I realize that many soon-to-be-ambitious photographers do purchase the camera as their first digital SLR), you might need to know that the lens aperture, or f/stop, is a ratio, much like a fraction, which is why f/2 is larger than f/4, just as 1/2 is larger than 1/4. However, f/2 is actually four times as large as f/4. (If you remember your high school geometry, you’ll know that to double the area of a circle, you multiply its diameter by the square root of two: 1.4.)
Lenses are usually marked with intermediate f/stops that represent a size that’s twice as much/half as much as the previous aperture. So, a lens might be marked f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, with each larger number representing an aperture that admits half as much light as the one before.
Shutter speeds are actual fractions (of a second), but the numerator is omitted, so that 60, 125, 250, 500, 1,000, and so forth represent 1/60th, 1/125th, 1/250th, 1/500th, and 1/1000th second. To avoid confusion, Canon uses quotation marks to signify longer exposures: 2", 2"5, 4", and so forth representing 2.0-, 2.5-, and 4.0-second exposures, respectively.
Most commonly, exposure settings are made using the aperture and shutter speed, followed by adjusting the ISO sensitivity if it’s not possible to get the preferred exposure; that is, the one that uses the “best” f/stop or shutter speed for the depth-of-field (range of sharp focus) or action stopping we want (produced by short shutter speeds, as I’ll explain later). Table 4.1 shows equivalent exposure settings using various shutter speeds and f/stops.
When the camera is set for P (Program) mode, the metering system selects the correct exposure for you automatically, but you can change quickly to an equivalent exposure by locking the current exposure (hold the shutter release down halfway, or press the * button), and then spinning the Main Dial until the desired equivalent exposure combination is displayed. You can use this standard Program Shift feature more easily if you remember that you need to rotate the dial toward the left when you want to increase the amount of depth-of-field or use a slower shutter speed; rotate to the right when you want to reduce the depth-of-field or use a faster shutter speed. The need for more/less DOF and slower/faster shutter speed are the primary reasons you’d want to use Program Shift. I’ll explain Program mode exposure shifting options in more detail later in this chapter.
In Aperture-priority (Av) and Shutter-priority (Tv) modes (or Fv mode when you opt to choose either aperture or shutter speed manually), you can change to an equivalent exposure using a different combination of shutter speed and aperture, but only by either adjusting the aperture in Aperture-priority mode (the camera then chooses the shutter speed) or shutter speed in Shutter-priority mode (the camera then selects the aperture). I’ll cover all these exposure modes and their differences later in the chapter.
The image shown in Figure 4.3, left, represents how a photograph might appear if you inserted the patches shown at bottom left into the scene, and then calculated exposure by measuring the light reflecting from the middle gray patch, which, for the sake of illustration, we’ll assume reflects approximately 12 to 18 percent of the light that strikes it. The gray patch also happens to be similar in reflectance to the background behind the subject. The exposure meter in the camera sees an object that it thinks is a middle gray, calculates an exposure based on that, and the patch in the center of the strip is rendered at its proper tonal value. Best of all, because the resulting exposure is correct, the black patch at left and white patch at right are rendered properly as well.
When you’re shooting pictures with your camera, and the meter happens to base its exposure on a subject that averages that “ideal” middle gray, you’ll end up with similar (accurate) results. The camera’s exposure algorithms are concocted to ensure this kind of result as often as possible, barring any unusual subjects (that is, those that are backlit, or have uneven illumination). The camera has four different metering modes, each of which is equipped to handle certain types of unusual subjects, as I’ll outline.
Figure 4.3, center, shows what would happen if the exposure were calculated based on metering the leftmost, black patch, which is roughly the same tonal value of the darkest areas of the subject’s hair. The light meter sees less light reflecting from the black square than it would see from a gray middle-tone subject, and so figures, “Aha! I need to add exposure to brighten this subject up to a middle gray!” That lightens the “black” patch, so it now appears to be gray.
But now the patch in the middle that was originally middle gray is overexposed and becomes light gray. And the white square at right is now seriously overexposed and loses detail in the highlights, which have become a featureless white. Our human subject is similarly overexposed.
The third possibility in this simplified scenario is that the light meter might measure the illumination bouncing off the white patch, which roughly corresponds to the subject’s blouse, and try to render that tone as a middle gray. A lot of light is reflected by the white square, so the exposure is reduced, bringing that patch closer to a middle-gray tone. The patches that were originally gray and black are now rendered too dark. Clearly, measuring the gray patch—or a substitute that reflects about the same amount of light, such as the standard Kodak gray card sold in many photo stores—is the only way to ensure that the exposure is precisely correct. (See Figure 4.3, right.)
As you can see, the ideal way to measure exposure is to meter from a subject that reflects 12 to 18 percent of the light that reaches it. If you want the most precise exposure calculations, the solution is to use a stand-in, such as the evenly illuminated gray card I just mentioned. But, because the standard Kodak gray card reflects 18 percent of the light that reaches it and your camera is calibrated for a somewhat darker 12 percent tone, you would need to add about one-half stop more exposure than the value metered from the card.
In some very bright scenes (like a snowy landscape or a lava field), you won’t have a mid-tone to meter. Another substitute for a gray card is the palm of a human hand (the backside of the hand is too variable). But a human palm, regardless of ethnic group, is even brighter than a standard gray card, so instead of one-half stop more exposure, you need to add one additional stop. That is, if your meter reading is 1/500th of a second at f/11, use 1/500th second at f/8 or 1/250th second at f/11 instead. (Both exposures are equivalent.)
Why are so many photographers under the impression that camera light meters are calibrated to the 18 percent “standard,” rather than the true value, which may be 12 to 14 percent, depending on the vendor? You’ll find this misinformation in an alarming number of places. I’ve seen the 18 percent myth taught in camera classes; I’ve found it in books, and even been given this wrong information from the technical staff of camera vendors. (They should know better—the same vendors’ engineers who design and calibrate the cameras have the right figure.)
The most common explanation is that during a revision of Kodak’s instructions for its gray cards in the 1970s, the advice to open up an extra half stop was omitted, and a whole generation of shooters grew up thinking that a measurement off a gray card could be used as-is. The proviso returned to the instructions by 1987, it’s said, but by then it was too late.
The light meters built into your camera are calibrated at the factory. But if you use a handheld incident or reflective light meter, you can calibrate it, using the instructions supplied with your meter. Because a handheld meter, of both the reflective and incident type, can be calibrated to the 18 percent gray standard (or any other value you choose), my rant about the myth of the 18 percent gray card doesn’t apply.
To calculate exposure automatically, you need to tell the camera where in the frame to measure the light (this is called the metering mode) and what controls should be used (aperture, shutter speed, or both) to set the exposure. That’s called exposure mode, and includes Program (P), Shutter-priority (Tv), Aperture-priority (Av), Flexible-priority (Fv), or Manual (M) options, plus Scene Intelligent Auto. I’ll explain all these next.
But first, I’m going to introduce you to the four metering modes. You can select any of the four if you’re working with P, Tv, Av, or M exposure modes; if you’re using Scene Intelligent Auto, Evaluative metering is selected automatically and cannot be changed. In Live View mode, only Evaluative and Center-weighted averaging modes can be selected.
Choose a metering mode by pressing the Q button and navigating to the Metering Mode icon, which is fifth from the top in the left column. (See Figure 4.4.) Then use either dial to select the mode you want, and press SET to confirm.
Available modes include:
The exposure zones used are linked to the autofocus system such that as the camera evaluates the measurements, it gives extra emphasis to the metering zones that indicate sharp focus. From this data, it makes an educated guess about what kind of picture you’re taking, based on examination of thousands of different real-world photos in the camera’s database. For example, if the top sections of a picture are much lighter than the bottom portions, the algorithm can assume that the scene is a landscape photo with lots of sky. This mode is the best all-purpose metering method for most pictures. I’ll explain how to choose an autofocus/exposure zone in the section on autofocus operation later in this chapter.
Note: Evaluative metering is sometimes described as a type of full-frame averaging. That’s absolutely incorrect. The camera intelligently considers the differences between the measured zones and then classifies what type of scene is being evaluated before calculating an exposure. Two subjects could have exactly the same average illumination but require quite different exposures, depending on the location of the bright, dark, and midtone areas of the scene.
You’ll find five semi-automatic and manual methods for choosing the appropriate shutter speed and aperture, including: Program (P), Shutter-priority (Tv), Aperture-priority (Av), and Manual (M). The fifth method, Flexible-Priority (Fv) can mimic any of the previous four. A sixth, Scene Intelligent Auto (A+), makes all the exposure calculations for you. To select one of these modes, just rotate the Mode Dial located at the top-right side of the R6 (see Figure 4.9, left), or press the MODE button on top of the R5 (see Figure 4.9, right) and use the QCD-1 or QCD-2 dial or the directional controls to cycle among the available exposure modes. Press the INFO button to toggle between still photography and movie modes.
Your choice of which exposure/shooting mode is best for a given shooting situation will depend on things like your need for more/less depth-of-field, a desire to freeze action or allow motion blur, or how much noise you find acceptable in an image. (Remember that exposure triangle at the beginning of the chapter?) Each of the camera’s exposure methods emphasizes one of those aspects of image capture or another. This section introduces you to all of them.
On first consideration, including an exposure mode with almost no user options might seem counterintuitive on your advanced camera, because it essentially transforms a sophisticated pro/enthusiast camera into a point-and-click snapshooter. Delve deeper, and you’ll discover that there is method in Canon’s madness, and that Scene Intelligent Auto is a lot more than a less versatile version of Program mode. The key is the Intelligent part of the mode’s nomenclature.
With P mode (discussed shortly), only the shutter speed and aperture are determined by the camera. You can change the metering mode, autofocus mode, white balance, and virtually all other settings. In Scene Intelligent Auto mode, the camera will analyze your scene, even to the extent of evaluating whether or not your subject is static or moving, and then intelligently choose optimum settings without any input from you. The settings the camera has to work with include:
Things that you can choose in Scene Intelligent Auto mode include:
Some specific Shooting menu options are available from the truncated four-tab menu system offered in Scene Intelligent Auto mode; there are also two Autofocus menu tabs. The available choices are described in more detail in Chapters 11 and 12.
In Av mode, you specify the lens opening used, and the camera selects the shutter speed. Aperture-priority is especially good when you want to use a particular lens opening to achieve a desired effect. Perhaps you’d like to use the smallest f/stop possible to maximize depth-of-field in a close-up picture. Or, you might want to use a large f/stop to throw everything except your main subject out of focus, as in Figure 4.10. Maybe you’d just like to “lock in” a particular f/stop smaller than the maximum aperture because it’s the sharpest available aperture with that lens. Or, you might prefer to use, say, f/2.8 on a lens with a maximum aperture of f/1.4, because you want the best compromise between speed and sharpness.
Aperture-priority can even be used to specify a range of shutter speeds you want to use under varying lighting conditions, which seems almost contradictory. But think about it. You’re shooting a soccer game outdoors with a telephoto lens and want a relatively high shutter speed, but you don’t care if the speed changes a little should the sun duck behind a cloud. Set your camera to Av, and adjust the aperture until a shutter speed of, say, 1/1000th second is selected at your current ISO setting. (In bright sunlight at ISO 400, that aperture is likely to be around f/11.) Then, go ahead and shoot, knowing that your camera will maintain that f/11 aperture (for sufficient DOF as the soccer players move about the field), but will drop down to 1/750th or 1/500th second if necessary, should the lighting change a little.
If the shutter speed in the viewfinder or on the Shooting Settings screen is blinking, that indicates that the camera is unable to select an appropriate shutter speed at the selected aperture and that overexposure (the 8000 is blinking) or underexposure (the 30 shutter speed is blinking) will occur at the current ISO setting. To correct overexposure, select a smaller aperture (if available) or choose a lower ISO sensitivity. Fix underexposure conditions by choosing a larger aperture (if possible) or a higher ISO setting.
That’s the major pitfall of using Av: you might select an f/stop that is too small or too large to allow an optimal exposure with the available shutter speeds. For example, if you choose f/2.8 as your aperture and the illumination is quite bright (say, at the beach or in snow), even your camera’s fastest shutter speed might not be able to cut down the amount of light reaching the sensor to provide the right exposure. Or, if you select f/8 in a dimly lit room, you might find yourself shooting with a very slow shutter speed that can cause blurring from subject movement or camera shake. Aperture-priority is best used by those with a bit of experience in choosing settings. Many seasoned photographers leave their camera set on Av all the time. The Safety Shift feature can be used to automatically override your selected aperture if the camera is unable to obtain a correct exposure. Safety Shift operates even when you’re using flash. I’ll show you how to configure that setting, which can also be used with P, Tv, and Fv modes, in Chapter 15.
When to use Aperture-priority:
If you use Av mode and select an aperture like f/11 or f/16, it’s your responsibility to make sure the shutter speed selected is fast enough to avoid losing detail to camera shake, or that the camera is mounted on a tripod. One thing that new landscape photographers fail to account for is the movement of distant leaves and tree branches. When seeking the ultimate in sharpness, go ahead and use Aperture-priority, but boost ISO sensitivity a bit, if necessary, to provide a sufficiently fast shutter speed, whether shooting hand-held or with a tripod.
Shutter-priority (Tv) is the inverse of Aperture-priority: you choose the shutter speed you’d like to use, and the camera’s metering system selects the appropriate f/stop. Perhaps you’re shooting action photos and you want to use the absolute fastest shutter speed available with your camera; in other cases, you might want to use a slow shutter speed to add some blur to a sports image that would be mundane if the action were completely frozen. Motor sports and track-and-field events particularly lend themselves to creative use of slower speeds, as you can see in Figure 4.11. Shutter-priority mode gives you some control over how much action-freezing capability your digital camera brings to bear in a particular situation.
You’ll also encounter the same problem as with Aperture-priority when you select a shutter speed that’s too long or too short for correct exposure under some conditions. I’ve shot outdoor soccer games on sunny fall evenings and used Shutter-priority mode to lock in a 1/1000th-second shutter speed, which triggered the blinking warning, even with the lens wide open.
Like Av mode, it’s possible to choose an inappropriate shutter speed. If that’s the case, the maximum aperture of your lens (to indicate underexposure) or the minimum aperture (to indicate overexposure) will blink. To fix, select a longer shutter speed or higher ISO setting (for underexposure), or a faster shutter speed/lower ISO setting (for overexposure), or use Safety Shift, mentioned previously.
When to use Shutter-priority:
Program mode (P) uses the camera’s built-in smarts to select the correct f/stop and shutter speed using a database of picture information that tells it which combination of shutter speed and aperture will work best for a particular photo. If the correct exposure cannot be achieved at the current ISO setting, the shutter speed or aperture indicator in the viewfinder will blink, indicating under- or overexposure. You can then boost or reduce the ISO to increase or decrease sensitivity.
The camera’s recommended exposure can be overridden if you want. Use the EV setting feature (described later, because it also applies to Tv and Av modes) to add or subtract exposure from the metered value. And, as I mentioned earlier in this chapter, you can change from the recommended setting to an equivalent setting (as shown in Table 4.1) that produces the same exposure but using a different combination of f/stop and shutter speed.
To accomplish this:
Your adjustment remains in force for a single exposure; if you want to change from the recommended settings for the next exposure, you’ll need to repeat those steps.
When to use Program mode priority:
Flexible-priority (Fv) takes a little getting used to, because, at least among veteran photographers, the shooting modes P, Tv, Av, and Manual (discussed later) are ingrained in our workflow. Fv almost seems counterintuitive until you’ve used it a few times and the realization comes that it is probably the most intuitive shooting mode of all. Flexible-priority gives you all four modes with full control of the three legs of the exposure triangle, all within a single setting.
In a nutshell, Fv, by default, acts like Program AE with Auto ISO activated. That is, the camera selects shutter speed, aperture, and ISO setting for you automatically. But you can elect to manually specify any or all of those three, and the camera’s shooting mode magically transforms from P to Av, Tv, or Manual. With the mode set to Fv:
Part of being an experienced photographer comes from knowing when to rely on your camera’s automation (including Scene Intelligent Auto or P mode), when to go semi-automatic (with Tv or Av), and when to set exposure manually (using M). Some photographers actually prefer to set their exposure manually most of the time, as the camera will be happy to provide an indication of when its metering system judges your settings provide the proper exposure, using the analog exposure scale at the bottom of the display (see Figure 4.12) and on the status LCD panel of the R5.
Manual exposure can come in handy in some situations. You might be taking a silhouette photo and find that none of the exposure modes or EV correction features give you exactly the effect you want. For example, when I shot the ballet dancer in Figure 4.13 in front of a mostly dark background highlighted by an illuminated curtain off to the right, there was no way any of my camera’s exposure modes would be able to interpret the scene the way I wanted to shoot it, even with Spot metering, which didn’t have a narrow enough field-of-view from my position. So, I took a couple test exposures, and set the exposure manually using the exact shutter speed and f/stop I needed. You might be working in a studio environment using multiple flash units. The additional flash are triggered by slave devices (gadgets that set off the flash when they sense the light from another flash, or, perhaps from a radio or infrared remote control). Your camera’s exposure meter doesn’t compensate for the extra illumination, and can’t interpret the flash exposure at all, so you need to set the aperture manually.
Because, depending on your proclivities, you might not need to set exposure manually very often, you should still make sure you understand how it works. Fortunately, the camera makes setting exposure manually very easy. Just use the MODE dial to select Manual exposure, then turn the Main Dial to set the shutter speed, and the QCD-1 to adjust the aperture. Press the shutter release halfway or press the AE Lock (*) button, and the exposure scale in the viewfinder shows you how far your chosen setting diverges from the metered exposure.
If you activate ISO Auto, you can add or subtract exposure compensation. Just tap the exposure scale at the bottom of the touch screen, use the Quick Control screen’s Exposure Compensation function in the graphical screen, or use the Exposure Compensation/AEB entry in the Shooting 2 menu.
When to use Manual exposure:
Another way of adjusting exposures is by changing the ISO sensitivity setting. Sometimes photographers forget about this option, because the common practice is to set the ISO once for a particular shooting session (say, at ISO 100 or 200 for bright sunlight outdoors, or ISO 800 when shooting indoors) and then forget about it. ISOs higher than ISO 100 or 200 are seen as “bad” or “necessary evils.” However, changing the ISO is a valid way of adjusting exposure settings, particularly with the Canon EOS R5/R6, which produces good results at ISO settings that create grainy, unusable pictures with some other camera models.
Indeed, I find myself using ISO adjustment as a convenient alternate way of adding or subtracting EV when shooting in Manual mode, and as a quick way of choosing equivalent exposures when in Auto or semi-automatic modes. For example, I’ve selected a Manual exposure with both f/stop and shutter speed suitable for my image using, say, ISO 200. I can change the exposure in one-third-stop increments by pressing the M-Fn button on top of the camera, highlighting ISO with the QCD-1, and then spinning the Main Dial one click at a time. The difference in image quality/noise at the base setting of ISO 200 is negligible if I dial in ISO 100 to reduce exposure a little or change to ISO 400 to increase exposure. I keep my preferred f/stop and shutter speed, but still adjust the exposure.
Or, perhaps, I am using Tv mode and the metered exposure at ISO 200 is 1/500th second at f/11. If I decide on the spur of the moment I’d rather use 1/500th second at f/8, I can press the M-Fn button, select ISO with the QCD-1, and spin the Main Dial to switch to ISO 100. Of course, it’s a good idea to monitor your ISO changes, so you don’t end up at ISO 1600 accidentally. ISO settings can, of course, also be used to boost or reduce sensitivity in particular shooting situations.
When not using Scene Intelligent Auto (which sets ISO automatically), the camera can set ISO speeds manually for stills. (In video mode, Auto ISO must be used in all modes except Manual exposure.) The ISO Speed Settings entry in the Shooting 2 menu allows you to specify what speeds are available and how they are used:
TIP The Lo, H1, and H2 settings enable ISO expansion, which may produce excessive noise, irregular colors, banding, and lower resolution. Use them with caution.
This setting has two modes. In Auto mode, the camera decides when the shutter speed is too low. You can fine-tune this by choosing Slower or Faster on the scale (–3 to +3) that appears. Or, you can manually select the “trigger” shutter speed, from 1 second to 1/8000th second.
TIP By default, both the exposure level increments (size of shutter speed or f/stop changes) are in 1/3-stop jumps. In the Custom Functions 1 menu, you can set exposure level increments to 1/3 or 1/2 stops, and ISO changes to 1/3- or 1-stop increments. The larger 1-stop step for ISO allows rapid switching through ISO 100, 200, 400, 800, and so forth.
Find yourself locked out of ISO settings lower than 200 or higher than 32,000? You’ve probably set Highlight Tone Priority to Enable in the Shooting 2 menu, as described in Chapter 11.
Visual image noise is that random grainy effect that some like to use as a special effect, but which, most of the time, is objectionable because it robs your image of detail even as it adds that “interesting” texture. Noise is caused by two different phenomena: high ISO settings and long exposures.
High ISO noise commonly first appears when you raise your camera’s sensitivity setting above ISO 3200. With Canon cameras, which are renowned for their good ISO noise characteristics, noise is usually fairly noticeable at ISO 6400 and above. At the H setting (ISO 102,400 equivalent [R5]/ISO 204,800 equivalent [R6]), noise is usually quite bothersome, which is why those lofty sensitivity ratings are disabled by default and must be activated with ISO expansion. This kind of noise appears as a result of the amplification needed to increase the sensitivity of the sensor. Because your sensor has twice as many green pixels as red and blue pixels, such noise is typically worse in areas that have red, blue, and magenta tones, because the green signals don’t have to be amplified as much to produce detail. While higher ISOs do pull details out of dark areas, they also amplify non-signal information randomly, creating noise.
A similar noisy phenomenon occurs during long time exposures, which allow more photons to reach the sensor, increasing your ability to capture a picture under low-light conditions. However, the longer exposures also increase the likelihood that some pixels will register random phantom photons, often because the longer an imager is “hot,” the warmer it gets, and that heat can be mistaken for photons. There’s also a special kind of noise that CMOS sensors like the one used in your camera are potentially susceptible to. With a CCD, the entire signal is conveyed off the chip and funneled through a single amplifier and analog-to-digital conversion circuit. Any noise introduced there is, at least, consistent. CMOS imagers, on the other hand, contain millions of individual amplifiers and A/D converters, all working in unison. Because all these circuits don’t necessarily process in precisely the same way all the time, they can introduce something called fixed-pattern noise into the image data.
Fortunately, Canon’s electronics geniuses have done an exceptional job minimizing noise from all causes in the camera. Even so, you might still want to apply the optional long exposure noise reduction that can be activated in the Shooting 4 menu. This type of noise reduction involves the camera taking a second, blank exposure, and comparing the random pixels in that image with the photograph you just took. Pixels that coincide in the two represent noise and can safely be suppressed. This noise reduction system, called dark frame subtraction, effectively doubles the amount of time required to take a picture, and is used only for exposures longer than one second. Noise reduction can reduce the amount of detail in your picture, as some image information may be removed along with the noise. So, you might want to use this feature with moderation. Some types of images don’t require noise reduction because the grainy pattern tends to blend into the overall scene.
To activate your camera’s long exposure noise reduction features, go to the Shooting 4 menu, as explained further in Chapter 11.
You can also apply noise reduction to a lesser extent using Photoshop or Canon Digital Photo Professional and when converting RAW files to some other format, using your favorite RAW converter, or an industrial-strength product like Noise Ninja (www.picturecode.com) to wipe out noise after you’ve already taken the picture.
Sometimes you’ll want more or less exposure than indicated by the camera’s metering system. Perhaps you want to underexpose to create a silhouette effect or overexpose to produce a high-key look. It’s easy to use the camera’s exposure compensation system to override the exposure recommendations, available in any non-automatic mode except Manual. There are three ways to make exposure value (EV) changes with the camera.
Note that this method has an advantage: you can specify automatic exposure bracketing from this screen just by rotating the Main Dial. I’ll explain bracketing in more detail next.
Bracketing is a method for shooting several consecutive exposures using different settings, as a way of improving the odds that one will be exactly right. Before digital cameras took over the universe, it was common to bracket exposures, shooting, say, a series of three photos at 1/125th second, but varying the f/stop from f/8 to f/11 to f/16. In practice, smaller than whole-stop increments were used for greater precision. Plus, it was just as common to keep the same aperture and vary the shutter speed, although in the days before electronic shutters, film cameras often had only whole-increment shutter speeds available. Figure 4.16 shows a typical bracketed series.
Today, cameras can bracket exposures much more precisely, and bracket white balance as well (using the WB Shift/Bkt entry found in the Shooting 3 menu and described in Chapter 11). While WB bracketing is sometimes used when getting color absolutely correct in the camera is important, autoexposure bracketing (AEB) is used much more often. When this feature is activated, the camera takes a series of shots, all at a different exposure value—one at the standard exposure, and the others with more or less exposure. In Av mode, the shutter speed will change, whereas in Tv mode, the aperture speed will change. The next sections will explain the parameters you can select.
In the Custom Function 1 menu, under the Number of Bracketed Shots entry, you can elect to bracket 2, 3, 5, or 7 shots:
Also in the Custom Function 1 menu, you’ll find a Bracketing Sequence entry, which allows you to specify the order in which the autoexposure bracketing series are exposed. Your choice will depend both on personal preference and what you intend to do with the bracketed shots. The options include:
The final relevant entry in the Custom Function 1 menu is Bracketing Auto Cancel. When you activate bracketing (in the Shooting 2 menu, described shortly), the camera continues to shoot bracketed exposures until you manually turn the bracket feature off, assuming you have this setting disabled. That’s a good thing. If you’re out shooting a series of bracketed exposures (especially for HDR), it’s convenient to have your bracket setting be “sticky” and still be active even if you turn your camera off. Some shooters like to bracket virtually everything and leave bracketing on routinely.
However, much of the time you’ll want to turn bracketing off, and you may not want to visit the Shooting 3 menu to deactivate it manually. Set Bracketing Auto Cancel to Enable in the Custom Function 1 menu, and bracketing is cancelled when you turn the camera off, change lenses, use the flash, or change memory cards. When this setting is set to Disable, bracketing remains in effect until you manually turn it off or use the flash. The flash still cancels bracketing, but your settings are retained.
You can choose the size of the jump between each of the bracketed exposures. To do that, you’ll need to visit the Expo. Comp./AEB entry in the Shooting 2 menu. There, you can select from +/- 1/3 to 3 full stops in 1/3-stop increments, by rotating the Main Dial. The next section provides instructions for producing a bracketed set.
Using autoexposure bracketing is trickier than it needs to be but has been made more flexible than with some earlier models. With the camera you are not limited to only three exposures (up to seven shots can be taken), and you can choose to bracket only overexposures or underexposures—a very useful improvement! Just follow these steps:
For example, in Figure 4.17, the left and right red highlighted bars are separated from the center bar by two marks, each representing 1/3rd stop, so the bracketing will produce one image at 2/3rds stop less than the zero point (the large center bar), one at the zero point, and one at 2/3rds stop more than that.
When the three bracket indicators aren’t separated, using the QCD-1, in effect, adds or subtracts exposure compensation. You’ll be shooting a “bracketed” set of one picture, with the zero point placed at the portion of the scale you indicated. Until you rotate the Main Dial to separate the three bracket indicators by at least one indicator, this screen just supplies EV adjustment. Also, keep in mind that the increments shown will be either 1/3 stop or 1/2 stop, depending on how you’ve set Exposure Level Increments in the Custom Function 1 menu.
NOTE AEB is disabled when you’re using flash, Multi Shot Noise Reduction, taking long time exposures with the Bulb setting, or if you have enabled the Auto Lighting Optimizer in the Shooting 2 menu (in which case the optimizer will probably override and nullify bracketing).
High dynamic range (HDR) photography is quite the rage these days, and entire books have been written on the subject. It’s not really a new technique—film photographers have been combining multiple exposures for ages to produce a single image of, say, an interior room while maintaining detail in the scene visible through the windows.
Suppose you wanted to photograph a dimly lit room that had a bright window showing an outdoors scene. Proper exposure for the room might be on the order of 1/60th second at f/2.8 at ISO 200, while the outdoors scene probably would require f/11 at 1/400th second. That’s almost a 7 EV step difference (approximately 7 f/stops) and effectively beyond the dynamic range of any digital camera.
Until camera sensors gain much higher dynamic ranges (which may not be as far into the distant future as we think), special tricks like Auto Lighting Optimizer and HDR photography will remain basic tools. You can create in-camera HDR exposures or shoot HDR the old-fashioned way—with separate bracketed exposures that are later combined in a tool like Photomatix or Adobe’s Merge to HDR Pro image-editing feature. I’m going to show you how to use both.
The in-camera HDR feature is simple, flexible, and surprisingly effective in creating high dynamic range images. It’s also remarkably easy to use. Although it combines only three images to create a single HDR photograph, and it’s not always as good as the manual HDR method, it’s a lot faster.
Figure 4.19 shows you a typical situation in which you might want to use this setting. When the exposure is set for the interior of this covered bridge, the foliage surrounding it is overexposed (upper left). When the exposure is adjusted to produce detail in the foliage, the interior of the bridge goes dark (lower left). HDR allows combining the detail from multiple images—not just the two shown at left, but as many as you want, if you combine them manually (as I’ll show you later) to get the image shown at right.
Here are some tips for using this feature:
You can locate HDR Mode in the Shooting 5 menu. Press the SET button, and you’ll be taken to the menu shown in Figure 4.20. This menu has five entries:
The Effect parameters generate five different special effects (see Figure 4.21):
HDR photography was, for a while, an incredibly popular fad. Everywhere you looked there were overprocessed, garish HDR images that had little relationship to reality. I’ve been able to resist the temptation to overdo my landscape and travel photography (unlike the deliberately awful example I created for Figure 4.22). The phony-looking skies, the unnatural halos that appear at the edges of some objects, and the weird textures are usually a giveaway. My rule of thumb is that, if you can tell it’s HDR, it’s been done wrong—unless your intent was to show off what HDR can do.
The technique does have its uses, especially if done subtly, or as a special effect. That’s what I was looking for when I shot Alastair Greene, guitarist for the Alan Parsons Project for Figure 4.23. I wanted an edgy, poster-like quality, and so applied HDR liberally, but with the hope that the effect might not be evident on first glance.
Although the camera does have its built-in HDR feature, you can usually get much better, more tasteful results if you create your high dynamic range images manually. You can use a tool such as Photoshop’s Merge to HDR Pro feature, a stand-alone HDR utility, or a third-party Photoshop plug-in.
When you’re using Merge to HDR Pro in Adobe Photoshop (similar functions are available in other programs, including the Mac/PC utility Photomatix [www.hdrsoft.com; free to try, $39–$99 to buy, depending on the version you select]) and Aurora HDR (www.skylum.com, $69), you’d take and combine several pictures. As I mentioned earlier, one would be exposed for the shadows, one for the highlights, and perhaps one for the midtones. Then, you’d use the Merge to HDR command (or the equivalent in other software) to combine all of the images into one HDR image that integrates the well-exposed sections of each version. You can use the camera’s bracketing feature to produce those images.
The next steps show you how to combine the separate exposures into one merged high dynamic range image. The sample images in Figure 4.24 show the results you can get from a three-shot (manually) bracketed sequence. The images should be as identical as possible, except for exposure. So, as with HDR mode, it’s a good idea to mount the camera on a tripod, use a remote release, and take all the exposures at once. Just follow these steps:
The next steps show you how to combine the separate exposures into one merged high dynamic range image.
What if you don’t have the opportunity, inclination, or skills to create several images at different exposures, as described? If you shoot in RAW format, you can still use Merge to HDR, working with a single original image file. What you do is import the image into Photoshop several times, using Adobe Camera Raw to create multiple copies of the file at different exposure levels.
For example, you’d create one copy that’s too dark, so the shadows lose detail, but the highlights are preserved. Create another copy with the shadows intact and allow the highlights to wash out. Then, you can use Merge to HDR to combine the two and end up with a finished image that has the extended dynamic range you’re looking for. (This concludes the image-editing portion of the chapter. We now return you to our alternate sponsor: photography.)
While you can often recover poorly exposed photos in your image editor, your best bet is to arrive at the correct exposure in the camera, minimizing the tweaks that you have to make in post-processing. However, you can’t always judge exposure just by simply looking at the preview image on your camera’s display before the shot is made, nor the review image in Playback. Ambient light may make the monitor difficult to see, and the brightness level you’ve set for the monitor and viewfinder in the Set-up menu can affect the appearance of the image.
Instead, you can use a histogram, which is a chart shown on the camera’s display that shows the number of tones that have been captured at each brightness level. Histograms are available in real time on your display as you shoot and in the review image during playback, but they are available only when enabled. I’ll show you how to enable histograms and select from among the various options available for histograms in Chapter 13. To view histograms in shooting mode or playback mode, press the INFO button until a screen with the histogram appears.
Histograms come in various flavors. Photographers are generally concerned only with two types: a brightness or luminance histogram, which deals only with the relative overall intensity of the tones in the image (see Figure 4.25, top), and a color histogram that displays the intensity of each individual color channel in a particular color space (see Figure 4.25, bottom). Photographers most often work with an RGB histogram that displays values for red, green, and blue pixels in an image, but other varieties exist, such as CMYK (for cyan, magenta, yellow, and black hues) and HSL/HSV (hue, saturation, and lightness/value), which are alternate ways of representing the RGB color space.
To get you up to speed with histograms, the next few sections will deal only with the brightness/luminance histogram variety.
Histograms help you adjust the tonal range of an image, the span of dark to light tones, from a complete absence of brightness (black) to the brightest possible tone (white), and all the middle tones in between. Because all values for tones fall into a continuous spectrum between black and white, it’s easiest to think of a photo’s tonality in terms of a black-and-white or grayscale image, even though you’re capturing those tones in three separate color layers of red, green, and blue.
Because your images are digital, the tonal “spectrum” isn’t really continuous: it’s divided into discrete steps that represent the different tones that can be captured. Figure 4.26 may help you understand this concept. The gray steps shown range from 100 percent gray (black) at the left, to 0 percent gray (white) at the right, with 20 gray steps in all (plus white).
Along the bottom of the chart are the digital values from 0 to 255 recorded by your sensor for an image with 8 bits per channel. (8 bits of red, 8 bits of green, and 8 bits of blue equal a 24-bit, full-color image.) Any black captured would be represented by a value of 0, the brightest white by 255, and the midtones would be clustered around the 128 marker. The actual information captured may be “finer” and record say, 0 to 4,094 for an image captured when the camera is set to 14 bits per channel for a RAW file (see Chapter 11 for more detail on that option).
Grayscale images (which we call black-and-white photos) are easy to understand. Or, at least, that’s what we think. When we look at a black-and-white image, we think we’re seeing a continuous range of tones from black to white, and all the grays in between. But, that’s not exactly true. The blackest black in any photo isn’t a true black, because some light is always reflected from the surface of the print, and if viewed on a screen, the deepest black is only as dark as the least-reflective area a computer monitor can produce. The whitest white isn’t a true white, either, because even the lightest areas of a print absorb some light (only a mirror reflects close to all the light that strikes it), and, when viewing on a computer monitor, the whites are limited by the brightness of the display’s LCD or LED picture elements. Lacking darker blacks and brighter, whiter whites, that continuous set of tones doesn’t cover the full grayscale tonal range.
The full scale of tones becomes useful when you have an image that has large expanses of shades that change gradually from one level to the next, such as areas of sky, water, or walls. Think of a picture taken of a group of campers around a campfire. Since the light from the fire is striking them directly in the face, there aren’t many shadows on the campers’ faces. All the tones that make up the features of the people around the fire are compressed into one end of the brightness spectrum—the lighter end.
Yet, there’s more to this scene than faces. Behind the campers are trees, rocks, and perhaps a few animals that have emerged from the shadows to see what is going on. These are illuminated by the softer light that bounces off the surrounding surfaces. If your eyes become accustomed to the reduced illumination, you’ll find that there is a wealth of detail in these shadow images.
This campfire scene would be a nightmare to reproduce faithfully under any circumstances. If you are an experienced photographer, you are probably already wincing at what is called a high-contrast lighting situation. Some photos may be high in contrast when there are fewer tones, and they are all bunched up at limited points in the scale. In a low-contrast image, there are more tones, but they are spread out so widely that the image looks flat. Your digital camera can show you the relationship between these tones using a histogram.
Your camera’s histograms are a simplified display of the numbers of pixels at each of 256 brightness levels, producing an interesting “mountain range” shape in the graph. Although separate charts may be provided for brightness and the red, green, and blue channels, when you first start using histograms, you’ll want to concentrate on the brightness histogram.
Each vertical line in the graph represents the number of pixels in the image for each brightness value, from 0 (black) on the left to 255 (white) on the right. The vertical axis measures that number of pixels at each level.
Although histograms are most often used to fine-tune exposure, you can glean other information from them, such as the relative contrast of the image. Figure 4.27, top, shows a generic histogram of an image having normal contrast. In such an image, most of the pixels are spread across the image, with a healthy distribution of tones throughout the midtone section of the graph. That large peak at the right side of the graph represents all those light tones in the sky. A normal-contrast image you shoot may have less sky area, and less of a peak at the right side, but notice that very few pixels hug the right edge of the histogram, indicating that the lightest tones are not being clipped because they are off the chart.
With a lower-contrast image, like the one shown in Figure 4.27, center, the basic shape of the previous histogram will remain recognizable, but gradually will be compressed together to cover a smaller area of the gray spectrum. The squished shape of the histogram is caused by all the grays in the original image being represented by a limited number of gray tones in a smaller range of the scale.
Instead of the darkest tones of the image reaching into the black end of the spectrum and the whitest tones extending to the lightest end, the blackest areas of the scene are now represented by a light gray, and the whites by a somewhat lighter gray. The overall contrast of the image is reduced. Because all the darker tones are actually a middle gray or lighter, the scene in this version of the photo appears lighter as well.
Going in the other direction, increasing the contrast of an image produces a histogram like the one shown in Figure 4.27, bottom. In this case, the tonal range is now spread over the entire width of the chart, but, except for the bright sky, there is not much variation in the middle tones; the mountain “peaks” are not very high. When you stretch the grayscale in both directions like this, the darkest tones become darker (that may not be possible) and the lightest tones become lighter (ditto). In fact, shades that might have been gray before can change to black or white as they are moved toward either end of the scale.
The effect of increasing contrast may be to move some tones off either end of the scale altogether, while spreading the remaining grays over a smaller number of locations on the spectrum. That’s exactly the case in the example shown. The number of possible tones is smaller, and the image appears harsher.
The important thing to remember when working with the histogram display in your camera is that changing the exposure does not change the contrast of an image. The curves illustrated in the previous three examples remain exactly the same shape when you increase or decrease exposure. I repeat: The proportional distribution of grays shown in the histogram doesn’t change when exposure changes; it is neither stretched nor compressed. However, the tones as a whole are moved toward one end of the scale or the other, depending on whether you’re increasing or decreasing exposure. You’ll be able to see that in some illustrations that follow.
So, as you reduce exposure, tones gradually move to the black end (and off the scale), while the reverse is true when you increase exposure. The contrast within the image is changed only to the extent that some of the tones can no longer be represented when they are moved off the scale.
To change the contrast of an image, you must do one of four things:
Of the four of these, the third—changing the contrast of the scene—is the most desirable, because attempting to fix contrast by fiddling with the tonal values is unlikely to be a perfect remedy. However, adding a little contrast can be successful because you can discard some tones to make the image more contrasty. However, the opposite is much more difficult. An overly contrasty image rarely can be fixed because you can’t add information that isn’t there in the first place.
What you can do is adjust the exposure so that the tones that are already present in the scene are captured correctly. Figure 4.28, top, shows the histogram for an image that is badly underexposed. You can guess from the shape of the histogram that many of the dark tones to the left of the graph have been clipped off. There’s plenty of room on the right side for additional pixels to reside without having them become overexposed. So, you can increase the exposure (either by changing the f/stop or shutter speed, or by adding an EV value) to produce the corrected histogram shown in Figure 4.28, center.
Conversely, if your histogram looks like the one shown in Figure 4.28, bottom, with bright tones pushed off the right edge of the chart, you have an overexposed image, and you can correct it by reducing exposure. In addition to the histogram, the camera has its Highlights option, which, when activated, shows areas that are overexposed with flashing tones (often called “blinkies”) in the review screen. Depending on the importance of this “clipped” detail, you can adjust exposure or leave it alone. For example, if all the dark-coded areas in the review are in a background that you care little about, you can forget about them and not change the exposure, but if such areas appear in facial details of your subject, you may want to make some adjustments.
In working with histograms, your goal should be to have all the tones in an image spread out between the edges, with none clipped off at the left and right sides. Underexposing (to preserve highlights) should be done only as a last resort, because retrieving the underexposed shadows in your image editor will frequently increase the noise, even if you’re working with RAW files. A better course of action is to expose for the highlights, but, when the subject matter makes it practical, fill in the shadows with additional light, using reflectors, fill flash, or other techniques rather than allowing them to be seriously underexposed.
A traditional technique for optimizing exposure is called “expose to the right” (ETTR), which involves adding exposure to push the histogram’s curve toward the right side but not far enough to clip off highlights. The rationale for this method is that extra shadow detail will be produced with a minimum increase in noise, especially in the shadow areas. It’s said that half of a digital sensor’s response lies in the brightest areas of an image, and so require the least amount of amplification (which is one way to increase digital noise). ETTR can work, as long as you’re able to capture a satisfactory amount of information in the shadows.
It’s easier to understand exposing to the right if you mentally divide the histogram into fifths (unfortunately, the camera’s histogram uses quarters instead). And, for the sake of simplicity and smaller numbers, assume you’re shooting in 14-bit RAW. Any 14-bit image can record a maximum of 16,383 different tones per channel. However, each fifth of the histogram does not encompass 3,277 tones (one-fifth of 16,383).
Instead, the right-most fifth, the highlights, shown in Figure 4.29, accounts for 8,192 different captured tones. Moving toward the left, the next fifth represents 4,096 levels, followed by 2,048 levels, 1,024 levels, and, in the left-most section where the deepest shadows reside, only 512 different tones are captured. When processing your RAW file, there are only 512 tones to recover in the shadows, which is why boosting/amplifying them increases noise. (The effect is most noticeable in the red and blue channels; your sensor’s Bayer array has twice as many green-sensitive pixels as red or blue.) Instead, you want to add exposure—as long as you don’t push highlights off the right edge of the histogram—to brighten the shadows. Because there are 8,192 tones available in the highlights, even if the RAW image looks overexposed, it’s possible to use your RAW converter’s Exposure slider (such as the one found in Adobe Camera Raw) to bring back detail captured in that surplus of tones in the highlights. This procedure is the exact opposite of what was recommended for film of the transparency variety—it was fairly easy to retrieve detail from shadows by pumping more light through them when processing the image, while even small amounts of extra exposure blew out highlights. (Note: I’ve rounded the numbers a bit for simplicity.) You’ll often find that the range of tones in your image is so great that there is no way to keep your histogram from spilling over into the left and right edges, costing you both highlight and shadow detail. Exposing to the right may not work in such situations. A second school of thought recommends reducing exposure to bring back the highlights, or “exposing to the left.” You would then attempt to recover shadow detail in an image editor, using tools like Adobe Camera Raw’s Exposure slider. But remember, above all, that this procedure will also boost noise in the shadows, and so the technique should be used with caution. In most cases, exposing to the right is your best bet.
The more you work with histograms, the more useful they become. One of the first things that histogram veterans notice is that it’s possible to overexpose one channel even if the overall exposure appears to be correct. For example, flower photographers soon discover that it’s really, really difficult to get a good picture of a red rose, like the one shown at left in Figure 4.30. The exposure looks okay—but there’s no detail in the rose’s petals. Looking at the histogram (see Figure 4.30, right) shows why: the red channel is blown out. If you look at the red histogram, there’s a peak at the right edge that indicates that highlight information has been lost. In fact, the green channel has been blown, too, and so the green parts of the flower also lack detail. Only the blue channel’s histogram is entirely contained within the boundaries of the chart, and, on first glance, the white luminance histogram at top of the column of graphs seems fairly normal.
Any of the primary channels—red, green, or blue—can blow out all by themselves, although bright reds seem to be the most common problem area. More difficult to diagnose are overexposed tones in one of the “in-between” hues on the color wheel. Overexposed yellows (which are very common) will be shown by blowouts in both the red and green channels. Too-bright cyans will manifest as excessive blue and green highlights, while overexposure in the red and blue channels reduces detail in magenta colors. As you gain experience, you’ll be able to see exactly how anomalies in the RGB channels translate into poor highlights and murky shadows.
The only way to correct for color channel blowouts is to reduce exposure. As I mentioned earlier, you might want to consider filling in the shadows with additional light to keep them from becoming too dark when you decrease exposure. In practice, you’ll want to monitor the red channel most closely, followed by the blue channel, and slightly decrease exposure to see if that helps. Because of the way our eyes perceive color, we are more sensitive to variations in green, so green channel blowouts are less of a problem, unless your main subject is heavily colored in that hue. If you plan on photographing a frog hopping around on your front lawn, you’ll want to be extra careful to preserve detail in the green channel, using bracketing or other exposure techniques outlined in this chapter.