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Tuesday, February 12, 2013

The Universal Exposure Strategy

How to harness the power within your camera and your eye to get perfect exposures for every kind of lighting situation

This Article Features Photo Zoom
The mound of data on the right side that represents the highlights should be almost, but not quite, butted up against the right side of the graph. If it's too far to the left, increase exposure a bit and try again.

If the mound of data is cut off by the right side of the graph, reduce exposure and try again. If your camera gives you a histogram with exposure simulation in Live View, you can adjust exposure before shooting until the highlights are almost, but not quite, clipped.

The most sophisticated way to use the Limiting Factor strategy is to spot-meter the highlights and open up an appropriate number of stops to place them just below clipping. To use this technique effectively, you need to know the dynamic range of your sensor, which you can establish by testing. I know, for example, that with my Canon EOS-1Ds Mark III, I can spot-meter a bright white cloud, open up three stops and still hold printable detail.

Aspen Panorama, Colorado Trail, San Isabel National Forest, Colorado. Randall used the Limiting Factor strategy to shoot a series of images that were stitched together to make the panorama.
3 Rembrandt Solution As contrast increases still further, it may be impossible to capture all the detail you want in both highlights and shadows in a single frame. My solution is a digital "graduated neutral-density" technique I call the Rembrandt Solution since it uses visual principles pioneered by the famed portrait painter.

I start by shooting two frames, one exposed for highlights, one for shadows. Next, I stack the two frames on top of each other in either Photoshop or Photoshop Elements (version 9 or later), with the dark (good highlights) frame on top. I add a layer mask to the top layer, then draw a white-to-black gradient on the mask to merge the two frames in a way that's exactly analogous, in terms of the end result, to using a physical, grad neutral-density filter in the field. For more details, see "The Rembrandt Solution" (Outdoor Photographer, September 2009).

4 HDR As a last resort, in the highest contrast situations, you may be forced to rely on HDR techniques to merge several bracketed frames in tone-mapping software. At a minimum, you'll need three frames shot at -2, 0 and +2 exposure compensation using the auto-bracketing feature on your camera. There are many good HDR programs, but my favorites continue to be HDRsoft Photomatix and Nik Software HDR Efex Pro.

The Universal Exposure Strategy lets you choose, after the fact, which of the four basic exposure strategies to employ. In a PhD situation, the middle frame of the bracketed set should be perfect. In fact, if you're completely confident it's a PhD scene, you don't need to bracket at all.


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