Photographic Image Quality:
Exposure

The human eye can adapt to see an amazingly wide range of light values between dark and bright. Cameras can't. In fact, cameras can't even “see” as wide a range of light values as we can represent in photographic prints and on a display screen. If an image is overexposed (too bright), the highlights (brightest areas) will be completely white or “blown out” with no detail. If the image is underexposed (too dark), the shadows (darkest areas) will be completely black or “blocked,” again with no detail. Sometimes it’s just not possible to avoid one or the other in a single exposure.

▸ To determine exposure within a usable range, photographers think of light in “stops,” sometimes called steps or exposure values (EV). Each stop represents twice as much light as the one below it and half as much light as the one above. To control the camera's exposure of an image, the photographer (or camera) can set three parameters: sensor response (called “ISO”), shutter speed, and lens aperture. Each of these is described by values that are designated in one-stop units. We'll put them together after we've described them individually.

ISO

ISO used to be a measure of film speed—the amount of light that a given film could respond to in a given amount of time. It was known as “ASA” back then, and the only way that a photographer could control it was to load a different roll of film. In the digital world, it is a measure of sensor response to light, variable in the camera. One-stop values are 100, 200, 400, 800, 1600, and even higher—higher being faster and thus allowing images to be taken in lower light.

▸ The problem is that the actual sensor response is the lowest number that the camera allows; higher numbers are attained by electronically boosting the signal to the camera's processor. Modern cameras are amazingly good at doing this, but higher numbers will still produce “noise,” seen as uneven colors or grainy textures. As we discussed earlier, smaller sensor pixels make this even more of a problem.

▸ Trivia: ISO stands for International Standards Organization; the earlier ASA was an American standard; there was also a German film speed standard called DIN.

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Shutter speed

Shutter speed controls the amount of time that the sensor is exposed to light coming through the lens. It is given in seconds or fractions of a second, with longer times obviously letting in more light. Many exposures in typical daylight conditions are taken at times around 1/100th second. Doubling the speed (for example, to 1/200th second) lets in half the light or one stop less, and halving it (1/50th second) lets in twice the light or one stop more. On most camera displays, the speed will be shown as a whole number instead of a fraction (100 = 1/100th), with longer times identified with a seconds mark (") or other symbol.

▸ Shutter speed also controls motion, either of the subject or of the camera itself. For hand-held shots, the shutter speed must be high enough to avoid camera shake; this depends in part on the length of the lens and in part (now) on the effectiveness of stabilized lenses. At some point, either the lens is long enough or the shutter speed slow enough that the camera must be mounted on a tripod or other support.

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Aperture (f-stop)

We described this earlier as a characteristic of the lens. To repeat: each stop value is approximately = [the previous one × √2]: 1, 1.4, 2, 2.8, 4, 5.6, and so on. A smaller aperture number is a larger opening, lets in more light and is said to be “faster”; a larger number is a smaller opening, lets in less light and is said to be “slower.” In the earlier discussion, we were concerned only with the maximum aperture. In shooting, we (or the camera) set the value to anything from the maximum to whatever minimum the lens offers, usually f/16 or f/22. The lens actually contains a diaphragm that closes down the amount of light admitted as the f-stop increases.

▸ Aperture also controls depth of field—the distance in front of and behind the plane of perfect focus that is still perceived as being acceptably sharp. As the lens opening gets larger (smaller number), depth of field is decreased and only part of the image (hopefully the most important part) is in focus. As the opening gets smaller (larger number), depth of field is increased. The effect of this control becomes much more pronounced as the focal length of the lens increases; it is also much more visible on large sensors than small ones.

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Setting the exposure

Almost all modern cameras let you choose “scenes” from which the camera can automatically determine exposure settings. But if you've read this far, you want to have more control of the camera based on what you are actually seeing in front of you. That means using one of the four shooting modes: Program, Aperture, Shutter (also called Time), or Manual. You select one of these with a dial (P-A-S-M or P-Av-Tv-M) or from a menu.

Program: the camera will choose both aperture and shutter speed, but not change the ISO or other settings that you have made in the camera. Use this when you are shooting quickly or the light is changing rapidly. Otherwise, A or S modes are best.

Aperture priority: you set the aperture; the camera chooses the shutter speed. Use this when you want to control depth of field, with a wider setting (f/2) making only the subject in focus and a smaller setting (f/16) making more of the scene sharp. Always check what the camera has set for shutter speed; you may have to adjust the aperture to get better overall results (for example, enough speed to hand-hold the camera without blurring the image).

Shutter (Time) priority: you set the shutter speed; the camera chooses the aperture. Use this when you want to control whether subject motion is blurred or sharp, for example to “stop” a runner (1/200th sec or higher) or show a waterfall as a silky blur (1/30th or slower). As above, also be sure that the speed is high enough to handhold, generally 1/[FF focal length of the lens] but possibly slower with IS lenses. If you need a longer exposure, mount the camera on a tripod or use some available platform (table, tree, car) to steady it. Again, check the aperture value that the camera has set to be sure it's what you want.

Manual: you set both the aperture and the shutter speed, with the camera's meter informing you how “correct” the exposure will be. This is most useful when you have the camera on a tripod, have plenty of time to set it up, and want to have complete control over how the image is exposed.

ISO: For any of the four modes, you should set the ISO first. (It's usually a separate control anyway.) Use the lowest setting possible for the lighting conditions: 100 for bright outdoor scenes, 200 or 400 for lower natural light or bright indoors. For truly low light, set the ISO higher—800, 1600, or more—but again be aware that you may have to deal with some noise in post-processing. Then when you are setting the aperture and/or the shutter speed, you may still have to adjust the ISO for the results you want (for example, raise the ISO to permit a higher shutter speed or smaller aperture).

Equivalent exposures: As you close the aperture, you can lengthen the shutter speed to allow the same amount of light to reach the sensor (and vice-versa). For example, the following combinations will be equivalent:

You can mentally “slide” either row left or right of the other one to get equivalent exposure pairs for brighter or darker light values.

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Exposure compensation

Remember that only one plane, perpendicular to the lens axis, can be in perfect focus at any one time. In the same way, the exposure setting determines how bright only one light value of the scene will be recorded. Lighter values in the scene will be recorded lighter by the sensor; darker values recorded darker.

▸ The total range of light values that the sensor can record is about seven stops. What the camera indicates as the “correct” value for you (or it) to set is actually the middle one of the seven. You can visualize it this way:

If the most important tones in your image should be recorded lighter than the meter indicates, increase the exposure; if they should be recorded darker, decrease the exposure. The exact way to do this varies from camera to camera. If you are in Manual mode, you just compensate manually rather than with a separate setting.

▸ For a much more detailed technical discussion of this topic, please see the Zone System Test gallery on my SmugMug site.

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HDR

There will be scenes in which the range of light values is simply far too great for the sensor to record—perhaps 10 stops or even more. For aesthetic reasons, you might choose to simply let all of the brightest values go completely white or all of the darkest values go completely black. But if you want your final image to show detail everywhere, your only choice is to take multiple exposures: one at the metered setting, one or more lighter, and one or more darker.

▸ This technique is called high dynamic range, or HDR, photography. The multiple images are combined with specialized post-processing software that produces a composite in which all values are within range to be printed or displayed on a screen. In order for the images to be aligned correctly, the camera must be mounted on a tripod or otherwise securely held between exposures.

▸ In addition to combining exposures, post-processing software allows for “tone mapping” the image to produce wide variations in color and exposure that can sometimes verge on the surreal. Some people equate “HDR” with these special effects, but they are different techniques that do not have to be used together.

▸ Some of the latest cameras can perform a version of HDR in-camera, but this is generally not as versatile as the method described above.

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