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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

Weminuche Wilderness Area, Colorado. By gradually changing how the background is illuminated so that lighter or darker foreground elements are placed against an opposing background, you can create the illusion of greater dynamic range. Called countershading, the technique is based on the same principles used by painters like Rembrandt centuries ago.
Landscape photographers face a fundamental challenge: how to compress the broad range of light intensities typically found in the real world into the much narrower range of tones that can be reproduced in a print. Four hundred years ago, painters like Rembrandt tackled high-contrast scenes using a technique called countershading to create the illusion of greater dynamic range in their paintings than actually existed. Photographers can achieve the same result with knowledgeable use of graduated neutral-density filters or Photoshop.

Let me start at the beginning. The maximum range of brightness levels in a print is about 50 to 1. This limitation has shackled artists from the cave painters of Lascaux to today’s high-tech digital photographers. The range of brightness levels in a print is limited by the amount of light reflected by even the brightest white paper and by the amount of light absorbed by the blackest ink available. The range of light intensities in the real world, in one scene, easily can be 10 times greater than what actually can be reproduced in a print.

Here’s where countershading comes in. Countershading is the technique of introducing a gradual change in the background illumination, let’s say from light to dark, so that light foreground elements placed against the dark part of the background gradient look brighter than they actually are. Countershading relies on two principles. The first is that our visual system is much more sensitive to abrupt changes in luminosity than gradual ones. The second is that surrounding a tone with a darker tone makes the original tone seem lighter; surrounding the original tone with a lighter one makes the original tone seem darker.

The human eye is very responsive to sudden changes of gradient density in the midtones of an image. FIGURE 1: The circle-gradient illusion demonstrates that by surrounding a tone with a darker one, the original appears lighter and vice versa.
Figure 1 gives a simple example of countershading called the circle-gradient illusion. Notice the apparent gradient inside the circle. Now cover up the background with a sheet of paper with a hole the size of the circle cut in it. Notice how the circle actually is a completely even tone. Simply by creating a gradient in the background, we’ve induced an apparent, opposite gradient in the foreground.

Now let’s take it further, to an illusion that shows how graduated neutral-density filters (split NDs, for short), whether physical or digital, can create the illusion of greater dynamic range than actually exists. Look at Figure 2, the Cornsweet Illusion. It shows two rectangles, one next to the other. The left rectangle should appear lighter than the right rectangle. Now cover the middle half of the strip, leaving the left and right rectangles exposed. Suddenly you see that, in reality, the left quarter of the strip is exactly the same density as the right quarter.

FIGURE 2: The Cornsweet Illusion. The outer parts of the rectangles are the same density, but look different because by simply putting two gradients next to each other, there’s an illusion of greater dynamic range than what actually exists. This is why graduated ND filters work so well in enhancing dynamic range where there’s a transition from highlight to shadow.
Here’s what’s going on. The left half of the illusion actually contains a gradient from midtone to lighter-than-midtone. The right half contains a gradient from darker-than-midtone back up to midtone. Our eyes are insensitive to the gradual change of density in the gradients, but very sensitive to the abrupt change of density in the middle. Here’s the crucial point: Merely by introducing two simple gradients, you can create the illusion of greater dynamic range than actually exists. The left and right rectangles of the illusion are, in reality, the same density, but they look different. The Cornsweet Illusion explains beautifully why you can use a split-ND filter with a gradual transition from dark to clear in a situation where the actual transition from highlight to shadow is abrupt and not only get away with it, but actually enhance the apparent dynamic range of your print.

Picture a typical split-ND situation, with brightly lit mountains and deeply shadowed foreground flowers as seen in the image below. You attach a split-ND with a gradual transition zone from dark to clear and position the middle of the transition zone over the sharp dividing line between shadowed flowers and sunlit peaks. Let’s analyze the situation in the captured file as we move from top to bottom along the filter. The uniform gray part of the filter uniformly darkens the upper part of the peaks. As the filter’s transition from dark to clear begins, the sunlit peaks actually become brighter as the amount of light absorbed by the filter gradually diminishes. At the shadow line, still beneath the transition zone of the filter, the shadow becomes darker than it otherwise would be because the filter’s transition zone hasn’t yet faded to perfectly clear. The bottom of the scene is unaffected because it’s behind the clear portion of the filter. A print of the image will show the illusion of greater dynamic range than actually exists.


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