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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

F. I wanted to preserve the warmth of the sunrise light on these aspen trees in front of Capitol Peak, so I walked close enough to spot-meter the sunlit side of the trunks and opened up one stop. That put the bright sky about two stops over midtone and the shadowed snow around midtone.
Don’t assume that you can just average the shadow and highlight readings and blaze away. Let’s say you meter some shadowed flowers and sunlit mountains and determine that you have a five-stop range—within the range of almost any DSLR. “Eureka!” you think. I’ll just pick an exposure in the middle of the range and it will be perfect.

Wrong! Flowers (or at least the green foliage around them) need to be exposed close to a midtone density to look right in a print. If you choose an exposure in the middle of that five-stop range, your flowers will be 2½ stops darker than midtone, and your highlights will be 2½ stops brighter, giving you very dark, muddy flowers and washed-out peaks. If you expose the flowers as a midtone, as you should, that puts the highlights 5 stops brighter than midtone—well beyond the range of even the best DSLRs. The solution in this case almost always will be a physical or virtual split-ND filter or some variation of HDR technique.

As this example shows, your exposure strategy needs to consider the proper density for the various elements in your scene, as well as the dynamic range of the scene and your sensor. But the starting point always is this: If you know both your sensor’s and your scene’s dynamic range, you can place the brightest important highlights just below your sensor’s limit. That, in turn, will give you the best shadow detail possible in a single capture that also preserves the highlights. And if the scene won’t fit gracefully inside your sensor’s dynamic range, you’ll learn that in the field when there’s still time to do something about it.

How To Measure Your Sensor’s Dynamic Range

To measure your sensor’s dynamic range, start by selecting near-white and near-black subjects that have some variation in tone. I chose a piece of dirty-white deck furniture and a dark shadow beneath that furniture. Now make a series of bracketed exposures of each subject.

For the near-white subject, start with the exposure the camera recommends when you fill the frame with the subject and open up a 1/3-stop for each successive frame by adjusting the shutter speed in manual-exposure mode. Your final frame should be 5 stops overexposed. For the near-black subject, start with the exposure the camera recommends and close down a 1/3-stop for each successive frame. Your final frame should be 5 stops underexposed.

Examine the photos in a RAW converter (I used Adobe Camera Raw). I adjusted the Exposure, Recovery and Fill Light sliders as necessary to get the best possible detail. As you examine lighter and lighter exposures, you’ll eventually find one where you can’t recover printable detail no matter what you do. This is the point where your sensor has saturated. As you examine darker and darker exposures, you’ll eventually find one where you can’t recover printable detail without generating unacceptable noise. This is the lower limit to your camera’s ability to hold shadow detail. For my Canon EOS-1Ds Mark III, I measured a range from +4 stops to -5 stops at the extreme limit, from near-white to near-black.

In addition to knowing the outer limits of your sensor, it’s also important to know the range with reasonable color and detail. To test this admittedly subjective parameter, I photographed some flowers, bracketing +3 and -3 stops in 1/3-stop increments. I used Camera Raw to adjust each exposure to match, as closely as possible, the middle exposure in the set (the one shot at the camera’s recommendation, which proved to be correct). That testing showed I could hold adequate color and detail over a range of about four stops (+2 to -2). Images overexposed more than two stops remained pale and washed out no matter what I did. Images underexposed more than two stops became muddy with excessive noise when I tried to bring them close to midtone.

While it’s always best to expose a midtoned part of a scene as a midtone in the original capture, this second test showed I could hold decent color in parts of the subject exposed within a range of +2 or -2 stops from the meter’s recommendation.

For more information and to see more of Glenn Randall’s photography, visit www.glennrandall.com.

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