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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

Labels: CamerasD-SLRs

This Article Features Photo Zoom

A. Overcast days are great for shooting intimate landscapes because of the soft, uniform lighting, but present an exposure challenge when you include the sky, which is always much brighter than the land. To capture this scene of Longs Peak from Bear Lake in Rocky Mountain National Park, I spot-metered the brightest clouds and opened up three stops. That put the aspen leaves at one stop under a midtone, within range for my sensor to record adequate detail at both ends of the tonal scale.

B. I calculated the exposure for this view of Delicate Arch in Arches National Park by spot-metering the bright, colorful clouds and opening up two stops, which put the shadowed rock at two or three stops darker than midtone. I used Photoshop’s Shadows/Highlights to recover good color at both ends of the tonal scale.
Sometimes your first capture is your only capture. Blow the exposure, and you’ve blown a once-in-a-lifetime opportunity. You can’t bracket high-action wildlife or outdoor sports—you’re all but guaranteed to get the wrong exposure at the decisive moment. Even landscape photographers like me need the ability to nail the exposure on the first try. I made one of my top-selling wildflower photos when the wind stopped just once during the fleeting seconds of perfect sunset light. Don’t “spray and pray,” I tell my students. Don’t turn on autobracketing, blaze away and hope for the best. Instead, I urge them, “Master the craft, and the art will follow.”

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.

C. For accuracy, don’t include the sun in the frame when metering backlit scenes such as this view of Partition Arch in Arches National Park. To get the best compromise exposure, I spot-metered the sky near the horizon in about the middle of the arch and opened up three stops. That put the sky farther from the sun (and horizon) at about two stops over midtone and the shadowed rock at two to three stops under midtone.
The best tool for measuring the dynamic range of a scene is a handheld spot meter. It allows you to mount your camera on a tripod and compose precisely, then leave your camera locked down while you meter the scene and calculate the exposure. My Sekonic L-608 spot meter also lets me memorize the exposure for any part of the scene, such as the darkest shadow. The meter then reads out the difference, in stops, between the memorized value and the value of whatever I meter next, such as the brightest highlights. That makes it effortless to measure a scene’s dynamic range.

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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