Tuesday, August 4, 2009
The Rembrandt Solution
What painting’s Grand Masters can teach today’s digital photographers
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.
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.
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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