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Photoshop - Curves Part 2





Curves can also be used on individual color channels to correct color casts in specific tonal regions. Often the color in an image is correctly balanced, but due to reflection or a light source with a varying temperature or color, you may see unwanted tints in a tonal region. Changing the white balance or adjusting the overall color would inadvertently harm the other tones. So we can selectively increase or decrease the amount of a color cast in the red, green, and blue channels to achieve perfect balance.


RGB Curves         Red Curves         Green Curves         Blue Curves
RGB Channel (Click to enlarge)     Red Channel (Click to enlarge)     Green Channel (Click to enlarge)      Blue Channel (Click to enlarge)

Any adjustments upward of the diagonal line in the red channel increase the red in the image. Lowering, below the diagonal line, increases the cyan. The other channels are the same: Upward in the green channel, green; lower, magenta. Upward in the blue channel, blue; lower yellow.

In the image to the right, there is a slight blueish cast in the color tone. Not to mention, the image is slightly washed out (improperly exposed).

As you can see, the sky is already quite white, so we won’t want to effect the highlights and above. By lowering the curve in the midtones and shadows, without effecting the highlights, we solve the exposure problem. Then we’ll get rid of the blue color cast: By lowering the blue in the low end, we eliminate the problem and the gowns go to black – as they should be.

If precise color adjustments aren’t required, simple color balance correction might be easier (IMAGE... Adjustments...Color Balance).


Rollover Image
Curves Adjustment Layer

Also, curves adjustment layers (LAYER...New Adjustment Layer... Curves) can be set to make curves only apply to a channel – such as Color and/or Luminosity – which allows for further, varied control. Another benefit is that it can make your adjustments more subtle through use of the opacity controls for the layer.

Here are some things to remember when using the curves tool:

  • Minimize use of the curves tool, as anything which stretches the image histogram increases the possibility posterization.
  • Avoid the use of the curves tool on an already altered image.
  • Perform curves on 16-bit images when possible. (IMAGE...Mode... 16 Bits/Channel)
  • For extreme levels of color correction, consider applying curves using LAB mode.