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I became aware of contrast masks when I was reading a book by Alain Briot, a photographer who lives in the Southwest. One image in particular impressed me greatly. Briot commented that he had needed to apply two contrast masks to tame the excessive contrast in the image. I decided that if contrast masks had anything to do with the quality of that image, I had to learn how to use them.
The only significant difference between these two photographs is that the second one has a contrast mask:
It's all a matter of taste, of course, but I prefer the image with the contrast mask. Contrast is fine, but the first image has too much of it.
If I knew who came up with this technique, I'd give him or her credit. If you do an internet search for contrast masks, you'll find a number of recipes for applying them. The recipes are all pretty much alike. Here's the approach I've taken to using.
(1) Pick a contrasty image to experiment with. In your layers-based program, create a duplicate image.
(2) Reduce the saturation on the duplicate layer to zero.
(3) To the duplicate layer, apply whatever the negative-image command is called in your software.
(4) Change the blend mode of the duplicate layer to 'overlay'.
(5) Using the levels adjustment, set the white and black points on the duplicate layer, and adjust the midtones until they seem about right. What you're seeing won't look good at this point. Ignore that fact and just try to get the tonal range roughly correct.
At this point, you can proceed in one of two ways. Here's what I prefer:
(6) Reduce the opacity of the duplicate layer until you like the result. I usually wind up setting the opacity at 30 – 40 percent. Sometimes I use an even lower opacity level. I seldom like the results with the opacity set any higher. On a few occasions, I've applied a second contrast mask in lieu of increasing the opacity of the first.
Some recipes for contrast masks suggest adding a gaussian blur to the duplicate layer. It's worth trying. Bear in mind, though, that you're working with a negative image. When you work on a negative image, the effects of gaussian blur and sharpening are reversed. Sharpening blurs the image and a blur sharpens it. If you add a gaussian blur to the duplicate layer, you may get an acceptable image at a higher opacity. You might experiment by setting the opacity of the duplicate layer at 80 percent, and trying a gaussian blur with a radius of perhaps five to fifteen. I've done side-by-side comparisons of using gaussian blur at a higher opacity and doing without the blur, at a lower opacity. The images look much the same to me, except that the version with blur often seems unpleasantly sharp. Haloes may appear. I prefer to keep my sharpening separate from my tonal adjustments.
Once you've applied a contrast mask, the result will often need some adjustment. For one thing, your black point won't be black any more, and your white point won't be white. I usually add a curves layer, and adjust the resulting tones to taste. Frequently I wind up restoring some—but not much—of the contrast that the mask removed. A contrast mask also tends to leave the image softer. Additional sharpening is usually required.
Consider what you've done with a contrast mask. By making the duplicate layer a negative image, you've given it dark tones where the background image is bright, and bright tones where the background image is dark. When you blend the two layers, you necessarily reduce the contrast in the image. The desaturation of the duplicate layer keeps the adjustment from affecting color. You only want to adjust the tones. The result is noticeably different from simply reducing the contrast through an ordinary adjustment.
Since a contrast mask lightens shadow areas, it seems to me that it ought to exacerbate noise in those areas. In practice, I haven't found that to be a problem, but it's worth keeping an eye on.
These are just my thoughts about contrast masks. I'd welcome others' experiences and views.
Obviously, not all images will benefit from a contrast mask. I take a lot of landscape photographs in the hard light of the American Southwest, though, and a significant proportion of those shots are improved by the application of a contrast mask. It's a good tool to know about, when you need it.