Tuesday, August 17, 2010
Max Out Your DSLR Sensor
Make the best exposure for a scene when you know how your camera will respond to the full spectrum, from highlights to shadows
To successfully photograph a high-contrast scene with a single capture, you need two key pieces of information: What’s the dynamic range of the scene? What’s the dynamic range of your sensor? Once you have that information, you can fit the range of tones in the scene into the range your sensor can capture as well as possible.
The dynamic range of the scene is just the difference, in ƒ-stops, between the darkest important shadow and brightest important highlight. Let’s say you meter the darkest shadow and get 1/60 sec. at ƒ/2.8. (In other words, you fill the frame with the darkest shadow—nothing else—and your camera recommends an exposure of 1/60 sec. at ƒ/2.8.) You meter the brightest highlight and get 1/60 sec. at ƒ/22. The dynamic range is six stops. (Count up from ƒ/2.8 in full-stop increments: ƒ/4, ƒ/5.6, ƒ/8, ƒ/11, ƒ/16, ƒ/22.) If you expose the scene at 1/60 sec. at ƒ/8 (midway between the shadow and highlight readings), then every part of the scene that meters 1⁄60 sec. at ƒ/8 will be rendered as a midtone. The darkest shadows will be three stops darker than midtone, and the brightest highlights will be three stops brighter than midtone.
If you don’t have a handheld meter, use your in-camera spot meter, coupled with manual exposure mode. Almost all DSLRs will display an analog exposure scale when placed in manual-exposure mode. Point the spot meter at a part of the scene you want to render as a midtone. Green grass and medium-gray rocks often are good starting points. Adjust the exposure until the pointer is in the middle of the analog scale, indicating a “correct” (meaning midtone) exposure for that part of the scene. Now point the spot meter at the darkest important shadow. Don’t adjust the shutter speed or aperture. The pointer will move down the analog scale and tell you, in stops, how much darker than midtone that part of the scene will be. Now point the spot meter at the highlights and see how much brighter than midtone they are. If need be, switch to a telephoto to get a narrower angle of view for your spot meter.
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