Hi guys, I thought I'd pass along a useful LAB color-correction technique that I use, that not many people know about. It's something that can't be done in RGB mode, at least without about a 15-step masking process that is not as accurate as a simple curves adjustment in LAB.
The technique is to reduce the saturation of low-saturation colors, without affecting the high-saturation colors. In RGB this would be called a "saturation curve," which is a tool that does not exist in Photoshop.
For example, in a photograph of a city, you want grey concrete things to be grey, not tinted with blue or purple or cyan or whatever, and you want your shadows to be neutral instead of blue or purple or red. But, you also want the blue things to be blue, and the cyan things to be cyan, etc. Using the standard RGB color tools, it can be difficult to remove the color from these neutrals while maintaining correct color balance and saturation overall.
Being able to do this is especially important for users of transparency films, because of tranny films' high sensitivity to color. Tranny films will almost always put too much color in the neutrals and blacks, which can give the image an over-saturated look. If you reduce saturation overall in RGB mode, that will take color out of the things that are supposed to be saturated, making the whole image look soft and dead. The technique I'm talking about here will allow you to take the color out of the low-saturation greys while leaving the colored objects untouched.
It's a simple curves adjustment in LAB, done only in the two color channels. Here it is: First go to the "a" channel, which controls the green-magenta spectrum. Lock down the center point (0,0), and then lock down a series of points up and down the line from 10 upwards, and -10 downwards. So you should have a point at 10, 10, then one at 14, 14, and one at 20, 20, and one at 24, 24. Then do the same thing with the negative values, placing points at -10,-10, -14,-14, -20,-20, etc.
Now place a point on -4, -4, and change the "output" number to 0, so the point is now at -4,0. Place another point on -8, -8, and change that output number to 0. That will have the effect of taking everything in the image that is slightly green (green values of 0 to 8), and taking all the green out, without affecting the things that are very green (with green values of 10 or more, which are locked down).
Now do the same thing to the positive side of the curve, which is the magenta side. Place a point on 4,4, and then change the output number to 0, and do it with 8,8 also. That will have the effect of taking everything in the image that is slightly magenta (with magenta values of 0 to 8) and neutralizing those to zero, without affecting any of the saturated magentas that have magenta values of 10 or more.
Now anything in the image that had a slight green or magenta cast is perfectly neutral grey, while all the saturated greens and magentas are untouched. Because of the way that LAB works, this tweak will apply to the whole tonal scale at once-- it will take a tiny green or magenta cast out of the highlights, midtones and shadows simultaneously; everywhere in the image that has a slight amount of green or magenta will be neutralized to a color value of zero.
Then you can do the exact same thing on the blue-yellow continuum on the "b" channel. The "b" channel adjustment is particularly useful because most tranny films throw too much blue into the neutrals and shadows. This tweak will fix that without taking any blue out of the things in the image that are supposed to be blue. It works especially well for images that have a lot of neutral greys, such as fog, cement, grey overcast skies, etc. Interestingly, after applying this curve, the colors frequently will appear more saturated, because you have neutralized the neutrals and now the colored objects will contrast more with the clean neutrals and the whole image will sparkle.
And of course you don't have to do all the colors at once; you can correct only the blues this way if you want, or only the magentas or whatever. Just lock down the other side of the curve and work on the side you want to neutralize.
If you do it as an adjustment layer you can fade it back to taste, and of course you also can adjust the curve to taste-- if 8,0 takes out too much color, then you can try 8,4 or so, which will be a softer version of this adjustment. You will have to flatten the image before converting back to RGB or the adjustment will be lost.
I usually do this as a final step to make sure the neutrals are nice and clean before resizing, sharpening and printing.
Cheers from Seattle,