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| A D-SLR’s flashing highlights feature blinks solid color to identify areas that are too bright for the sensor to record detail (top). Properly reduced exposure (bottom) makes them disappear. |
The story is altogether different when you shoot in RAW mode, which retains the sensor's full 4,096 levels of brightness. While a JPEG emerges from your D-SLR essentially as a finished photograph, with RAW files and software you can make substantial adjustments in the brightness of a captured image without compromising its overall quality. This post-processing flexibility allows you to apply an exposure strategy to digital photography that I call Expose to the Right, or ETTR for short. ETTR is simple to apply, but to do so you need to refer to the histogram -- that mountain-peak graphic on your camera's LCD screen representing how the scene's tones are distributed. If you've set the camera to display it, the histogram will appear whenever you review a picture you've just shot. (See sidebar, right.)
In a histogram, the dark values are to the left and the light values are to the right. With the ETTR technique, you deliberately bias your exposure, changing f-stop, shutter speed, and/or exposure compensation so that as much of the mountain range as possible appears to the right side of the histogram. (Many digital point-and-shoots show you a "live," pre-exposure histogram, but with a D-SLR you have to make an exposure first so that you can study the histogram, make the needed adjustments, then reshoot.) The extra exposure moves the dark tones away from the noise floor, producing cleaner-looking shadows. By moving the dark tones higher up on the curve, it also places them where more data space is available. The result: smoother and more pleasing gradation.
When you Expose to the Right, though, be warned: The image that comes into your RAW converter program will probably appear too light. This doesn't matter as long as you haven't "blown out" any important highlights; with the exception of shiny chrome, the brightest bits of white clouds, and the like, nothing should be beyond the right edge of the histogram. (The "flashing highlights" indicator in most D-SLRs blinks to show you which parts of the picture itself are overexposed.) Once you've imported the image, you can rein in its overall brightness to produce the result you want.
Exposing to the Right doesn't work in every photographic situation. A scene with an unusually wide range of brightness levels that fill the entire histogram (the mountain peaks range all the way across the graph) provides no room for such adjustment. Biasing the exposure to the right (that is, giving any extra exposure) will sacrifice important detail in light areas. In such cases a normal, unadjusted exposure is better. Also keep in mind that some scenes may actually exceed the brightness-recording range of your camera's sensor. (That is, peaks representing important tonal values bunch up at both sides of the graph.) In this situation you have two options. If you're shooting handheld and can only make a single exposure, use a split neutral density filter to handle the adjustment in-camera. If you're working on a tripod, take multiple bracketed exposures and composite them in your image-editing software.
In a way, the ETTR approach is digital photography's equivalent of the film world's Zone System, in which a negative's exposure and development are manipulated to produce the desired tonal scale in the final print. In the Zone System, a negative is exposed for a specific degree of development that will "place" a scene's brightnesses on the best part of its tone curve. Likewise, using ETTR in conjunction with RAW software's capabilities allows you to improve image quality and produce better final prints.
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