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Tuesday, August 4, 2009

The Rembrandt Solution

What painting’s Grand Masters can teach today’s digital photographers

This Article Features Photo Zoom

FIGURE 3: The white
paper in the shade is actually darker than the black ink in the sun even though, on first glance, the opposite seems true. The human eye analyzes contrast in isolated parts of an image differently than it does when looking at the whole image.

There’s no need to perfectly mask out the incorrectly exposed portions of the two exposures; in fact, it’s better not to because the transition between the highlight and shadow regions will look more natural if you don’t. Perfectly masking out the too-dark shadows and blown highlights on the two layers would lead to a transition from cool shadow to warm highlight that had a color change but no density change—a highly unnatural result.

In high-contrast situations, our visual system separates the scene into various “zones” and analyzes the local contrast in each zone independently of other zones. For a scene to look natural, the local contrast must look right in each zone. By definition, the local contrast is correct in the correctly exposed regions of both the highlight and shadow exposures. By using what’s essentially a Cornsweet illusion-like pair of tonal gradients to merge the two correctly exposed regions of the images, natural-looking local contrast is maintained in both the shadows and highlights.

Put another way, you always get the best contrast and color in regions of your image exposed close to a midtone. By using a split ND, you can position the highlights and shadows close to a midtone and marry the two regions in a way that our visual system finds believable.

Singh-Ray ND Graduated Filter
For an illustration of how our visual system analyzes local contrast rather than global contrast in high-contrast scenes, check out Figure 3. Examine the two thin rectangular regions I’ve outlined. The white paper in shade looks significantly brighter than the black ink in the sun. How could it be otherwise? Isn’t white paper always brighter than black ink? Now cover up everything but the two thin rectangles. You’ll see that the “white” paper is actually much darker than the “black” ink.

So why not just use Photomatix Pro or another HDR software package for all high-contrast scenes? Current-generation HDR software doesn’t always guess perfectly what the local contrast should be in each region of the image, sometimes producing an unnatural result. Today’s HDR software also tends to produce halos and oversaturated highlights if pushed too hard. Further processing in Photoshop is usually necessary. Any motion in the scene creates problems when you merge the images.


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