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The lens aperture and shutter can control the light striking the light sensor, but to do the job properly, they need to be carefully controlled. A sunny day may be 10,000 times brighter than a dark room, yet the camera's light sensor is called upon to register minute differences in each. At one time, photographers had to guess at the correct aperture and shutter speed settings. Light meters that measure the brightness of individual scenes gave them a tool to objectively evaluate the proper shutter and aperture settings. Automatic exposure systems directly link the light meter to the exposure-regulating controls of the camera so you don't have to worry about adjusting the settings at all.
Making an automatic exposure system work requires the camera measure the brightness of the scene you want to photograph. So you need to measure the brightness of the scene. However, brightness is a property not of each scene but of each part of a scene. As long as the world isn't a uniform gray, some parts of a scene will be brighter than others. This inevitable difference in brightness is good—without it we wouldn't see anything. But it poses a difficult question for exposure metering: Where should you measure brightness?
Answering that question is tough because it has no one correct answer. Certainly there are obvious choices: Measure everything, measure the brightest spot, average the brightest and darkest areas, measure only the center, and so on. Although almost any strategy results in an image, often acceptable, none will always produce the best possible image. Finding a way of evaluating image brightness in the greatest number of situations has challenged camera designers since they first glued selenium cells on the fronts of their products to make automatic exposure systems.
The advent of the digital camera alters only the mechanics of the situation. Digital cameras (and video cameras, for that matter) can, in theory, measure the brightness at every pixel in an image. But the camera designer must choose how to combine those signals to control the aperture and shutter.
The issue is not only which pixels to sample but what importance to assign to the signal from each one—that is, the weighting to assign to each of potentially a few million brightness measurements. The guiding factor is as much aesthetic as engineering, however. Through the years, a number of schemes have been tried, increasing in complexity as technology has allowed. The primary choices are three (in order of introduction): full-scene, spot, and matrix, lumped together as metering patterns.
Full-scene is simply an average of the brightness of the entire scene, which works in a surprisingly large number of cases. But it's also fooled into the wrong setting in many cases. Backlighting—when a person faces the camera with the sun behind him—usually results in a silhouette, leaving the subject's smile lost in the darkness.
Spot metering looks only at a central spot of the image, usually the most important part of the image. By measuring only the face in a backlit scene, for example, it ignores the bright sun and lets you see the expression on your subject's face. But a bright or dark spot in the center of the image can fool the spot meter. (Most spot-based systems let you compensate by making a measurement on the most important part of your subject, locking the setting, and then letting you compose the image and make the exposure.
Matrix metering subdivides the image into several areas of importance, applies a weight to each, and then integrates the values to come up with an exposure setting. The most advanced form of matrix metering uses computer-based intelligence to deduce what you're trying to photograph—not quite a canned aesthetic sense but no more than a step away. Camera-makers have discovered that certain patterns of brightness—for example, bright around the edges but dark in the center—are trademarks of common photographic situations. The example would indicate a backlit subject, which the camera can identify and compensate for. The best digital cameras use matrix metering. Each camera-maker has developed its own set of patterns and algorithms that it believes best cover most photographic situations.
Metering only creates information. The next issue is what to do with the information that the metering system develops. The shutter speed and aperture interact, so you have to choose the best setting of each for your particular photographic situation. You have two ways of dealing with the choices—let the camera deal with them or do it yourself (in other words, automatic or manual exposure control).
Manual exposure is the old-fashioned way. You read the meter (which may be inside the camera and displayed in the viewfinder of a modern digital camera) and then adjust both the shutter speed and lens aperture to match its recommendations. Old match-needle metering requires you simply to adjust the camera so that the needle of its light meter moves to the center of its display; newer cameras may ask you to make adjustments until a red LED turns green, or some such nonsense.
For the most part, manual exposure systems existed because camera-makers had not yet learned how to make them automatic. Manual exposure still gives you the utmost in control, however. It allows you to control the "depth of field" of your photographs, the amount of blurring from image movement, and similar aesthetic pictorial features. If you have the least pretense toward making artistic photographs, you'll want a camera that allows you to step backward to manual exposure control.
Automatic exposure is not a single miracle, it's several. Auto-exposure systems may operate in any of three distinct modes and may even allow you to choose which to use. These modes include the following:
In any case, most cameras set off flashing lights, beeps, or sirens to warn when you try to photograph a scene out of the range of the automatic exposure system's capabilities to ensure a proper exposure. (A few snotty cameras may even prevent you from taking a photo that they think they cannot properly expose.) Usually adjusting the parameter that's left to manual control (the aperture or shutter that has priority) will resolve these difficulties unless the scene is just too dark.
The compromise between automatic and manual exposure is to handle things automatically but give you a veto. That is, let the camera find its own way but allow you to take over when things get too challenging.
To allow you to override the automatic exposure system and fine-tune the exposure of your images, most cameras provide exposure-compensation controls. These are also called backlight switches or something similar in recognition of the situation most often requiring compensation: when the source of light is behind the actual subject, making the background substantially brighter than the foreground. The exposure-compensation control simply tells the camera to overexpose the overall image to bring the actual subject into the proper exposure range. This overexposure control is also useful for other special situations, such as snowy ski scenes and bright sandy beaches. Although advanced matrix metering systems supposedly cope with this sort of lighting, a camera with exposure compensation gives you a degree of control without sacrificing general automatic operation.
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