Correct exposure is not the most important aspect of taking a picture. How well you frame or compose the image really makes or breaks a photograph. The exposure can be a little off and the picture can still look good, but if the composition is awry, there's not a lot you can do to save it. Fortunately, compositions are not chosen by the camera. In most cases, however, exposure settings are. You can do a great deal to ensure that your exposure settings mirror what you want to get out of your image.
As you recall from Chapter 2, exposure is defined as the amount of light striking the sensor. Think of each photosite or pixel on the sensor as a little bucket that catches photons of light as they pass through the lens. The photons must fill the bucket to a particular point (called a threshold) before that pixel will register its part of the image. Raising or lowering the threshold mark on the bucket decreases or increases the working sensitivity of the sensor. It doesn't make any difference whether the photons fill the bucket slowly or quickly, or whether they arrive in large streams or in a trickle. A pixel won't register an image until the threshold point is reached.
Figure 3.2 shows a fanciful representation of those buckets. The yellow drops at top represent sunlight, which contains red, green, and blue light, until all but one of those hues are removed by the colored filter mosaic placed atop a standard CCD sensor. The red, green, and blue photons continue past their respective filters to collect in the "bucket." If the bucket is filled at least to the threshold point (shown as a gray band around the bucket), that photosite registers that color. The amount of photons collected past the threshold point represents the density of the image for that photosite for that color. Once the bucket is completely filled, it can collect no more photons.
Figure 3.2. Photons (shown as drops) must fill the sensor's "buckets" to the threshold mark for an image to register.
The camera's shutter speed determines the interval used to flood the buckets with photons. A short shutter speed (such as 1/500th second) lets photons flow only for a brief period of time. A longer shutter speed (such as 1/60th second) lets the photons reach the sensor for a longer period. Figure 3.3 represents an exposure that is relatively short, so that photons can collect in the bucket for only a brief period of time.
Figure 3.3. A short shutter speed lets photons reach the sensor for only a brief period of time.
The lens's aperture size, or f-stop, controls how many photons are admitted at once. A small f-stop (on a non-SLR digital camera, the smallest stop will be around f8; SLR lenses can go as small as f22 to f45) restricts the number of photons. A larger f-stop (such as f2.8) lets a larger number of the available photons through at once. Figure 3.4 shows how a smaller f-stop restricts the number of photons that can reach the bucket.
Figure 3.4. Choosing a smaller f-stop reduces the number of photons that can reach the sensor, too.
Your sensor's photosite buckets can be filled quickly (with a short shutter speed and a large lens opening) or slowly (with a longer shutter speed and a smaller lens opening). Each faster shutter speed increment or f-stop lets in one-half the amount of light as the previous setting, so you can produce exactly the same exposure by switching to a smaller f-stop if you make the shutter speed twice as long (and vice versa).
These equivalent values are called Exposure Values, or EVs for short, and are consistent for any particular ISO setting on your camera. For example, a value of EV14 might represent 1/250th at f8 with a particular film. It would also represent all the other combinations of shutter speed and f-stop that would produce the same equivalent exposure, as shown in Table 3.1.
Shutter speed | f-stop | Shutter speed | f-stop |
|---|---|---|---|
1/30th second | f22 | 1/500th second | f5.6 |
1/60th second | f16 | 1/1,000th second | f4 |
1/125th second | f11 | 1/2,000th second | f2.8 |
1/250th second | f8 | 1/4,000th second | f2 |
Your digital camera might have EV values that you can set. It might also have an EV+ or EV- dial, which lets you correct the automatic exposure by an amount you specify. For example, choosing EV+0.5 will add half an EV to your exposure, which would be the same as either opening the aperture by half an f-stop or by making the shutter speed 50 percent longer. (It's easy for a digital camera to deal with fractions of an f-stop or fractional shutter speeds, because everything is done electronically.)
If you need to adjust your exposure manually or override your camera's settings to produce a darker or lighter image, it's easy to do by adjusting the EV+/EV- dial. Most cameras will let you add plus or minus 2 or 3 EV in either 1/2 or 1/3 increment steps. If you're making the correction manually using the shutter speed or aperture controls, just remember:
- Opening the lens aperture one stop (to a smaller f-stop number, such as going from f8 to f5.6) doubles the amount of light reaching the sensor.
- Closing the lens aperture one stop (to a larger f-stop number, such as going from f5.6 to f8) halves the amount of light.
- Choosing a shorter shutter speed (from 1/250th to 1/500th second, for example) cuts the amount of light reaching the sensor in half.
- Choosing a longer shutter speed (from 1/500th to 1/250th second) doubles the amount of light.
- Cutting the exposure in half with either aperture or shutter speed while doubling the exposure with the other control keeps the exposure exactly the same.
When an Exposure Isn't
Because you can halve or double an exposure by changing either the f-stop or shutter speed, these equivalent exposures are said to be reciprocal. It shouldn't make any difference which control you or your camera use to change exposure; the result should be the same whether you choose a smaller f-stop or a shorter shutter speed to reduce exposure (or opt for a larger f-stop and longer shutter speed to increase exposure). Unfortunately, both the film and digital worlds have to contend with a phenomenon known as reciprocity failure. Film and sensors react to very, very short and very, very long exposures in a non-linear way so that, say, an exposure of 60 seconds at f16 will not necessarily be exactly equal to an exposure of one second at f2. If you're experimenting with long time exposures at night, say, to capture a dim city street scene, you might find that you've seriously underexposed your image.
Reciprocity failure can happen at the other end of the time scale, too, with very brief exposures, although even the 1/4,000th second speed of some cameras is not likely to produce the effect. However, some electronic flash units can produce very brief exposures of 1/50,000th second or shorter in automatic mode at close distances.
In either case, if you're using your camera's automatic exposure features, you should use the exposure compensation control to allow extra exposure. Or, you can switch to manual mode and make additional exposures for a longer period of time using slower shutter speeds, or using a larger f-stop.
Because you can halve or double an exposure by changing either the f-stop or shutter speed, these equivalent exposures are said to be reciprocal. It shouldn't make any difference which control you or your camera use to change exposure; the result should be the same whether you choose a smaller f-stop or a shorter shutter speed to reduce exposure (or opt for a larger f-stop and longer shutter speed to increase exposure). Unfortunately, both the film and digital worlds have to contend with a phenomenon known as reciprocity failure. Film and sensors react to very, very short and very, very long exposures in a non-linear way so that, say, an exposure of 60 seconds at f16 will not necessarily be exactly equal to an exposure of one second at f2. If you're experimenting with long time exposures at night, say, to capture a dim city street scene, you might find that you've seriously underexposed your image.
Reciprocity failure can happen at the other end of the time scale, too, with very brief exposures, although even the 1/4,000th second speed of some cameras is not likely to produce the effect. However, some electronic flash units can produce very brief exposures of 1/50,000th second or shorter in automatic mode at close distances.
In either case, if you're using your camera's automatic exposure features, you should use the exposure compensation control to allow extra exposure. Or, you can switch to manual mode and make additional exposures for a longer period of time using slower shutter speeds, or using a larger f-stop.
How Exposure Is Calculated
Until you start thinking about it, calculating a correct exposure doesn't seem to be all that complicated. Just measure the amount of light reaching the sensor and figure out what combination of shutter speed and f-stop is required to allow exactly the optimal amount of light for the exposure.
Unfortunately, that sort of calculation won't work. It doesn't take into account the fact that subjects reflect different amounts of light. Imagine, for a moment, that you're taking a photo that contains a white cat and a dark gray cat. In the finished photo, you want the white cat to appear white and the dark gray cat to appear to be dark gray. Yet, the white cat might reflect five times (or more) as much light as the gray cat. Any exposure based on the white cat will cause the gray cat to appear to be much darker, almost black. Conversely, any exposure based only on the gray cat will make the white cat completely washed out, with no detail whatsoever. The proper exposure in this case is an average of the tones in the scene, a happy medium in which both the white cat and gray cat are properly exposed.
Light-measuring devices achieve this average by assuming that all the objects being measured average a standard value of 18 percent gray. The 18 percent figure was arrived at as a value that represents a gray tone that's halfway between black and white as perceived by film and, later, digital sensors (although not necessarily by human vision; there is still some controversy over this point). The assumption is an important one, because the camera or light meter has no way of knowing whether it is pointed at an object that is highly reflective, or one that doesn't reflect much light at all. Instead, calculations are made as if the object were reflecting 18 percent gray. It's the job of the photographer or automatic exposure system to make adjustments when that is not the case, using some sort of programming or compensation. Figure 3.5 shows a continuous gray scale with the approximate 18 percent reflectance point marked.
In most cases, your camera's light meter will do a good job of calculating the right exposure, especially if you use the exposure tips in the next section. But if you want to double-check or feel that exposure is especially critical, take the light reading off an object of known reflectance. Photographers sometimes carry around an 18 percent gray card (available from any camera store) and, for critical exposures, actually use that card, placed in the subject area, to measure exposure.
Until you start thinking about it, calculating a correct exposure doesn't seem to be all that complicated. Just measure the amount of light reaching the sensor and figure out what combination of shutter speed and f-stop is required to allow exactly the optimal amount of light for the exposure.
Unfortunately, that sort of calculation won't work. It doesn't take into account the fact that subjects reflect different amounts of light. Imagine, for a moment, that you're taking a photo that contains a white cat and a dark gray cat. In the finished photo, you want the white cat to appear white and the dark gray cat to appear to be dark gray. Yet, the white cat might reflect five times (or more) as much light as the gray cat. Any exposure based on the white cat will cause the gray cat to appear to be much darker, almost black. Conversely, any exposure based only on the gray cat will make the white cat completely washed out, with no detail whatsoever. The proper exposure in this case is an average of the tones in the scene, a happy medium in which both the white cat and gray cat are properly exposed.
Light-measuring devices achieve this average by assuming that all the objects being measured average a standard value of 18 percent gray. The 18 percent figure was arrived at as a value that represents a gray tone that's halfway between black and white as perceived by film and, later, digital sensors (although not necessarily by human vision; there is still some controversy over this point). The assumption is an important one, because the camera or light meter has no way of knowing whether it is pointed at an object that is highly reflective, or one that doesn't reflect much light at all. Instead, calculations are made as if the object were reflecting 18 percent gray. It's the job of the photographer or automatic exposure system to make adjustments when that is not the case, using some sort of programming or compensation. Figure 3.5 shows a continuous gray scale with the approximate 18 percent reflectance point marked.
Figure 3.5. A reflectance of 18 percent approximates a middle gray tone in most scenes.
In most cases, your camera's light meter will do a good job of calculating the right exposure, especially if you use the exposure tips in the next section. But if you want to double-check or feel that exposure is especially critical, take the light reading off an object of known reflectance. Photographers sometimes carry around an 18 percent gray card (available from any camera store) and, for critical exposures, actually use that card, placed in the subject area, to measure exposure.
Bracketing
Bracketing is the practice of taking several photos using different exposures to ensure that at least one will be optimal for your image. For example, you might take one picture at the "correct" exposure (as determined by your camera's exposure meter), plus one with half as much exposure, and one with twice as much exposure (change either the f-stop or shutter speed as appropriate). At least, that's how it was done in the old days before cameras had as many automatic features as they do now. Today, many electronic cameras (both film and digital) have a setting for automatic bracketing. The camera will snap off several exposures for you at different settings.
Indeed, digital cameras can bracket parameters other than exposure, such as white balance, to improve your chances of getting a photo that is fine-tuned for your specific light source.
Bracketing is the practice of taking several photos using different exposures to ensure that at least one will be optimal for your image. For example, you might take one picture at the "correct" exposure (as determined by your camera's exposure meter), plus one with half as much exposure, and one with twice as much exposure (change either the f-stop or shutter speed as appropriate). At least, that's how it was done in the old days before cameras had as many automatic features as they do now. Today, many electronic cameras (both film and digital) have a setting for automatic bracketing. The camera will snap off several exposures for you at different settings.
Indeed, digital cameras can bracket parameters other than exposure, such as white balance, to improve your chances of getting a photo that is fine-tuned for your specific light source.
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