Adjusting Contrast and Color With Curves
The curves adjustments control is one of the most powerful tools offered by Photoshop and other image editing programs, such as Lightroom and Paint Shop Pro. This one command allows you to lighten or darken an image, adjust contrast, and adjust color balance. Used on an Adjustment Layer, it allows you to make completely reversible, non destructive edits that can be removed or adjusted at any time. You can even use curves for very precise, controllable dodging and burning by selecting an area of your photo and making a curves adjustment layer from that selection! Dodging and burning is a term from the traditional darkroom that refers to lightening or darkening specific areas without affecting the rest of the photograph.
I also have a YouTube video tutorial for Photoshop Curves.
You can use curves to affect only one area of the tone curve, such as the lights, by grabbing the curve in the light area and pulling up or pushing down. You can increase contrast by pulling the lights up (making them brighter) and pulling the darks down (making them darker). You can decrease contrast the opposite way, by pulling the lights down and pushing the darks up as the following examples show. If you only want to effect the lights, or only the darks, move the part of the line you want to affect. You’ll see that, for exemple, if you lighten the lights, the darks do lighten a bit too. You can correct that by placing a second point in the darks and moving the line back closer to where it started. The grid helps you do that. You can actually have many points on the curve for very fine control of the tonal rendering.
The same techniques we used with the RGB curve to alter brightness and contrast are used in the color curves to color balance. For example, if you have a picture that’s got great color, except that the lights are too red, you can use the red curve to pull down the lights area of the curve, while moving the midtone and dark parts back to the normal places so they don’t change color. Only the lights will become less red, nothing else will change! The examples below show how color corrections work. I've exaggerated the changes to make the effect easier to see.
You can also color correct and tone correct with a single curves adjustment. When you flip back and forth between the RGB curve and the three color curves, the adjustments you did to each curve are kept so that you can change brightness, contrast, and color all before you hit the OK button. When you do hit ok, all the curve changes happen together so you don’t have to open the curves box several times.
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