Evaluative Metering The DSLR divides the image area into many segments to generate accurate exposures in a wide range of situations. It's best for general landscapes.
Nikon D300S Meter-Mode Switch
Let's start with the basics. An exposure meter measures the amount of light that strikes it and, based on this data and the ISO you've programmed into it, provides you with a series of shutter-speed/ƒ-stop combinations that will produce proper exposure for the metered subject or scene. So goes the theory.
"Proper" exposure is the one that produces the results you want in your photograph. If you want your image to look like the scene you photographed, with detail from the brightest important area to the darkest important area and a natural range of tones, the camera's built-in exposure meter will deliver that—assuming the scene has an "average" mix of tones and its brightness range doesn't exceed the film's or sensor's capabilities. And, of course, artistic expression may cause you to vary from a literal rendering of the scene.
If you give a scene too much exposure, the resulting photo will be too light—the darkest tone will be gray instead of black and the brighter areas will be blown out. If you give a scene too little exposure, the resulting photo will be too dark—the middle tones will be dark gray or black and the brightest areas will be muddy gray instead of white. So learning how your camera's metering system handles various scenes and exposure situations is an important part of being a good photographer.
The main idea behind reflected-light exposure meters (which includes those built into cameras) is that they're calibrated so that whatever you take a reading from will be reproduced as a medium tone in the photo if you expose according to the meter reading. If you take a reading from a medium-toned gray card and expose accordingly, the card will appear medium gray in the resulting photo. If you take a reading of a black card, less light will reach the meter, so it will call for more exposure, and the result will be the black card appearing as a medium-gray tone in the photo. If you meter a white card, more light will reach the meter, so it will call for less exposure, and the white card will appear medium gray in the resulting photo.
If all three cards appear in the photo, what's the correct exposure? The one from the gray card. If you expose for that (by metering it), the gray card will appear medium gray in the resulting photo, the black card will appear darker and the white card lighter. If you meter the black card and expose accordingly, the black card will appear medium gray in the resulting photo, and the gray and white cards will be too light. If you meter the white card and expose accordingly, the white card will appear medium gray in the resulting photo, and the gray and black cards will be too dark.
The meters in DSLRs (and most film SLRs) read the light through the lens. This provides a number of advantages over an external exposure meter. First, the meter reads only the light from the image area, regardless of lens focal length. Second, you don't have to carry an external meter. Third, the built-in TTL meter is connected to the camera's controls, so exposure automation is possible. The camera can set the shutter speed and/or aperture automatically based on the meter reading. While it can be "fooled" by particularly bright or dark subjects or scenes, TTL metering is a good thing. It speeds up shooting, makes things easier for the less experienced photographer and gives even the experienced shooter a good starting point.
Most cameras offer three basic metering modes: multi-segment, center-weighted and spot.
With multi-segment metering, the system divides the image area into a number of segments (Nikon's D7000 uses a 2,016-pixel RGB metering sensor; more common are double-digit numbers of segments), then uses brightness data from those segments, subject-position data from the AF system and, with some systems, even color data to calculate the best exposure for the subject/scene. While it's not right all the time, multi-segment metering is a good one to use for general shooting, when you're shooting quickly or when you don't want to think about metering—it will give the best exposures for the greatest number of scenes/situations.
Center-weighted metering reads the entire image area, but places most of the emphasis on the central portion (where the subject often is), helpful when the subject is against a brighter or darker background. Center-weighted metering was popular in the early days of TTL metering, but multi-segment produces better exposures more often, and spot metering provides more control when you want it. Some cameras let you adjust the size of the center-weighted area. Center-weighted metering is good for portraits.
Spot metering reads a small area of the scene (generally indicated by a small circle in the center of the viewfinder) and allows knowledgeable users to control exposure more precisely, based on the fact that whatever you take a spot reading from will be reproduced as a medium tone in the photograph. Meter an important portion of the subject or scene, decide how you want it to appear in the photo (medium, darker or brighter), and adjust the exposure accordingly. If you meter a darker area and want it to appear dark, give less exposure than the spot reading calls for; if you meter a bright area and want it to appear bright, give more exposure than the reading calls for.
How much more or less exposure should you give? That depends on how much brighter or darker than medium tone you want the metered area to appear. The Zone System provides a very precise way of controlling exposures using a spot meter when shooting film and can be adapted to digital to a degree, as well. You could start by metering a medium-gray card, exposing per the meter reading, then shooting additional images giving more exposure and less exposure in 1/3- or 1/2-stop increments out to 4 or 5 stops beyond the initial exposure. That will give you an idea of how your camera responds to changes in exposure.
You can use the spot meter to determine the approximate brightness range of the scene before you—just meter the brightest important part and note the reading, then read the darkest important part and note that reading, and see how far apart the readings are. For example, if the meter called for a shadow exposure of 1/15 sec. at ƒ/8 and a highlight exposure of 1/2000 sec. at ƒ/8, the range is 7 stops.
Digital Exposure Advantages
Digital cameras offer two big exposure advantages over film cameras: instant playback and histograms. With a digital camera, you can check the image right after you shoot it on the camera's LCD monitor. If it's over- or underexposed, you'll know at a glance and can reshoot it then and there.
You also can display a histogram for any image. A histogram is just a graph showing the distribution of tones in the image, from darkest at the left end to brightest at the right end. Each histogram will be different, as each image is different. A low-key image containing mainly dark tones will have most of the histogram toward the left side, while a high-key image consisting mostly of bright tones will have the histogram loaded to the right.
The main thing to keep in mind about the histogram is that you don't want it running off either end, especially the right end. If it runs off the right end, you've blown out the highlights. If it runs off the left end, you've lost the shadows. A quick glance at the histogram will tell you instantly if either has happened. If the histogram runs off the right side, make another shot, giving less exposure. Generally, you'll want the histogram to run near to, but not off, the right edge. If the histogram runs off both edges, the scene's brightness range is too great to record detail throughout, and you'll have to choose whether you want to lose highlight or shadow detail (generally, it's better to save highlight detail and let the darkest areas go black, but that's up to you, the artist, to decide). (Note that the histogram is for a camera-processed JPEG image, even if you're shooting RAW, and thus isn't necessarily an accurate representation of the RAW file data. Likewise, the image you play back on the LCD monitor is an in-camera JPEG, even if you're shooting RAW.)
Spot Metering Although unforgiving, spot metering is the best choice for situations where you need to base the exposure on a particular portion of a scene or subject. It's also ideal for checking the overall range in a scene and applying the Zone System. With most cameras, spot metering can be linked to the active AF point; it needn't always be the center point.
In the early days of built-in TTL metering, it was all center-weighted average. Today, multiple metering modes and exposure algorithms provide for greater accuracy and control, as well as higher success rates with a range of subjects and scenes. It's difficult to apply hard-and-fast rules to one metering mode versus another. By understanding how each mode works, you can determine the best choice for any scene. Generally, evaluative metering is a good all-purpose mode. The other modes come into play where the light or subject presents some challenges. Always keep in mind that meters are tools; they don't think and they don't know what you've previsualized. Our spot-metering example shows a scene where you need to use the meter to evaluate the range and then set the appropriate exposure for the effect that you want.
You can try using the camera's range-expanding tools (Canon's Auto Lighting Optimizer and Highlight Tone Priority, Nikon's Active D-Lighting, Sony's D-Range Optimizer, etc.) or built-in HDR, if available, to retain more detail from highlight through shadow. You also can shoot a bracketed series of exposures (using a tripod to hold the camera in position), then merge the highlight data from the underexposed image and the shadow data from the overexposed image in Photoshop or other image-editing software, or using HDR software.
Exposure compensation lets you direct the camera to give more or less exposure than the built-in meter suggests. This is handy if your camera consistently under- or overexposes slightly, or when shooting a particularly difficult scene that "fools" the built-in meter. If your image appears too dark or too light when you check it on the LCD monitor, dial in some exposure compensation and shoot another image. (Keep in mind that some metering systems break down the scene into many segments, and if you move the camera slightly between the original shot and the compensated one, that can change the metered exposure; with such cameras, it's best to use a tripod to avoid this possibility.)
All DSLRs offer four basic exposure modes, and some offer additional ones. The basic exposure modes are programmed AE, shutter-priority AE and aperture-priority AE, plus manual.
Diagrams and three-dimensional graphs for the Canon EOS-1D Mark IV show the relative weighting of evaluative-, partial- and spot-metering areas.
In programmed AE ("Program" mode), the camera sets the shutter speed and the aperture for "proper" exposure (as per the camera's meter and programming). This is handy when you want to concentrate on lighting and composition.
In shutter-priority AE, you set the shutter speed you want to use (to "freeze" or blur an action subject, for example), and the camera will automatically set the corresponding aperture for proper exposure.
In aperture-priority AE, you choose the aperture you wish to use (a small one to provide great depth of field or a wide one to isolate the subject from the background, for example), and the camera will automatically set the corresponding shutter speed for proper exposure.
Some DSLRs have an exposure safety-shift feature. When this is activated, the camera will maintain the shutter speed you selected in shutter-priority mode or the aperture you selected in aperture-priority mode as long as possible. If the lighting conditions are such that proper exposure isn't possible at the selected shutter speed or aperture, the camera will adjust the selected speed or aperture as needed to maintain proper exposure.
Manual Exposure Control
While automatic exposure control offers quick and convenient operation, it's not perfect. Automation can be fooled, and the camera doesn't necessarily know what you want your image to look like. So a number of serious photographers use manual exposure mode, setting the shutter speed and aperture themselves based on the built-in camera meter, a handheld meter or their own experience. The benefits of manual exposure control include getting exactly what you want in your image and not having to worry about the meter being fooled by bright or dark backgrounds as a moving subject passes them. The big drawback to manual exposure control for outdoor photography is that you really have to pay attention on partly cloudy days, as the exposure will keep changing as clouds block and then reveal the sun.
Normally, you select the ISO setting you wish to use. For most purposes, that will be the lowest one that will let you get your shot (i.e., use a fast enough shutter speed and appropriate aperture in the extant lighting conditions). Lower ISO settings generally deliver better image quality than higher ones, much as lower ISO films generally produce better image quality than faster films.
As a convenience, some DSLRs have an auto ISO feature, in which the camera will adjust the ISO automatically to maintain the best image quality. In program or aperture-priority AE, this is generally the lowest ISO that will permit a handholdable shutter speed with the focal length in use, based on the "handholding rule of thumb": Use a shutter speed no slower than the reciprocal of the focal length; for example, 1/50 sec. with a 50mm lens. Bear in mind that it may not always provide a suitable shutter speed—1/300 sec. isn't optimal for doing birds in flight with a 300mm lens, for example.