OP Home > How-To > Shooting > Max Out Your DSLR Sensor


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

D. To get the best exposure for this winter scene of Capitol Peak, I spot-metered the sunlit rock and opened up two stops, which put the white aspen trunks in the foreground at about one stop under midtone. To bring back the richness of the sunset light, I decreased exposure by three-quarters of a stop in Adobe Camera Raw, then used Fill Light to restore shadow detail. If the foreground subject had been darker in tone, the dynamic range of the scene would have been considerably greater.
Some analog exposure scales only extend +2 or -2 stops from midtone. If the dynamic range exceeds that, you’ll need a different approach. Start by pointing the spot meter at the shadow and setting the “correct” exposure. The pointer will be in the middle of the analog scale. (This isn’t the exposure you’ll actually use to make the picture.) Then point the meter at the highlight, reset the “correct” exposure and calculate the difference. To avoid the mental math, you can count clicks as you adjust the aperture and/or shutter speed. Each click will represent a change of a 1/3- or ½-stop, depending on the settings you’ve chosen in your camera’s menus. If it takes 18 clicks, each representing a 1/3-stop, to go from the correct exposure for the shadows to the correct exposure for the highlights, then the range is 6 stops. It’s usually easiest to adjust shutter speed alone, since its range is much wider than the aperture range.

If you have a quick-release tripod head, you can compose your shot, lock down the tripod controls, remove the camera from the tripod head, meter the scene and reattach the camera without disturbing your composition.

Knowing the dynamic range of a scene is meaningless unless you also know how wide a range your sensor can capture. Unfortunately, camera manufacturers rarely publish a number, which means most photographers don’t know the dynamic range of their sensor—but should. In the days when nearly all outdoor photographers shot transparency film, the dynamic range of the capture medium was essentially fixed. If you were shooting Velvia, for example, you knew you had a range of about 5 stops. You could place the brightest important highlights 2.3 stops brighter than a midtone and expect bright white highlights with barely printable detail. If you placed the darkest important shadows at 2.7 stops darker than midtone, you would get dark shadows just shy of pure black.

E. This 700-year-old limber pine near Lake Haiyaha in Rocky Mountain National Park doesn’t get sunrise light at anytime of year. I placed the sunlit rock on Hallett Peak at two stops over midtone, which put the foreground tree one or two stops under midtone, using Photoshop’s Shadows/Highlights to darken the highlights and brighten the shadows.
Unlike film, the dynamic range of sensors isn’t a fixed, known quantity. In fact, it varies from camera to camera. Older, less expensive DSLRs might have a range of six stops. Newer and more expensive models usually have a greater dynamic range, as do cameras with larger sensors—perhaps as much as nine stops. The only way to determine that range accurately is by performing a simple test. In brief, it involves selecting near-white and near-black subjects and making a series of bracketed exposures, then examining the images in a RAW converter to determine the sensor’s ability to hold detail in progressively lighter and darker captures. (See the sidebar for full details.) For my Canon EOS-1Ds Mark III, I measure a range from +4 stops to -5 stops at the extreme limit, from near-white to near-black. Further testing showed I could hold reasonable color and detail over a range of about four stops (+2 to -2).

Armed with this information, I can approach high-contrast scenes with confidence. Last fall, for example, I photographed Longs Peak in Colorado framed by colorful aspen. Using a split-neutral-density filter, either virtual or physical, to tame the bright sky would have rendered the tops of the trees too dark. The fluttering leaves made HDR techniques unworkable. But my testing had showed I could spot-meter the bright gray clouds and place them three stops over a midtone. In other words, I could meter the clouds at 1/30 sec. at ƒ/16, let’s say, then open up three stops to ¼ sec. at ƒ/16. I then metered the leaves at 1½ sec. at ƒ/16, or one stop darker than the exposure I actually used (¼ sec. at ƒ/16). The final exposure gave me light gray clouds in my digital file and excellent detail in the leaves. When a shaft of sunrise light found an unexpected hole in the clouds and spotlighted the leaves, I was ready.


Add Comment


Popular OP Articles