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Working with Curves in RGB - or
Photoshop Curves to make Adjustments to Tone in RGB
category: digital photography and image editing
You can do things to tone with Curves in one adjustment that would take several Levels adjustments with masks to accomplish, but first you have to realize that Curves doesn't really work in RGB, and what to do about it.
Color correction, yeah, but that's different. This is about tone; if you want to darken the sky and bring out detail in some forground shadows, you can't just Ctrl-Click on the image and start moving the curve in RGB like you can in Lab or Grayscale mode. Why? Well, because when you move a point on the curve you move all three color channels the same amount, so what was pure red becomes a little less red and now contains some green and/or blue.
That's easy; just set the blend mode of the Curves Adjustment Layer to Luminosity and you won't change the color at all. It's just like making your adjustment in Lab color, but you don't have to flatten the image. Great, but there's one more thing; in order for Photoshop to place the points on the curve accurately when you Ctrl-Click on the image, the image has to be grayscale with all three channels reporting the same number. Oops.
OK, no problem, just create a temporary Hue Saturation Adjustment Layer and desaturate the image. Now you can get accurate point placement with the added benifit of losing those distracting color shifts. That is to say, make your tonal adjustments in grayscale, check it out in color, and delete the desaturate layer when you're happy with your changes.
He said on 05.11.05 @ 01:04 PM CST
