Curves are hard to use.

Started Apr 25, 2014 | Discussions thread
suddie1215 Veteran Member • Posts: 3,818
Re: Curves are hard to use - a couple of basic points

What you've said isn't entirely accurate.  The Curves tool is designed to alter BOTH tones and colors.  If you use the composite RGB Curves line you will alter mainly tonal values; to limit the adjustment so that ONLY tones are affected, you need to change the layer blend mode to Luminosity.

A classic S-Curve to Increase Contrast using the composite RGB curve:

But you can also alter color by clicking on the drop-down box and selecting the individual Red, Green or Blue channels.  From there its a matter of simple Additive Color Theory.

  • Increasing the Red curve adds red to the image, decreasing it adds Cyan (or decreases red).
  • Increasing the Green curve adds green to the image; decreasing it adds Magenta
  • Increasing the Blue Curve adds Blue to the image; decreasing it adds Yellow.

To restrict the adjustment to only color, change the layer blend mode to Color.

Here's an example showing the individual Green channel selected in the drop-down list, and the Green curve pushed up to increase that color in the image.

Finally here's a depiction of adjustments to both the composite RGB curve and the individual color curves.  While this can be done in the Normal layer blend mode, its advised to do tonal and color adjustments separately so you can restrict the changes by switching layer blend modes.

In modern versions of Photoshop there's no need to make round-trips to Lab Mode and introduce the risk of quantization errors; you simply change the layer blend mode to Luminosity and only the tonal values will be modified.  Also, duplicating a layer (and  unnecessarily doubling  the file size) is not required since you can simply add a Curves Adjustment Layer by clicking on the Adjustment Layers icon in the Layers palette.

gordonpritchard wrote:

Curves are designed to be used to alter tonal contrast rather than color. If you use curves on an RGB image both color and cotrast will be affected. To adjust contrast only, which is the point of the tool, either convert the image to Lab mode and select only the L channel to apply a change with the curves tool, or, duplicate your RGB image into a new layer and set that layer to "Luminosity" then apply a change in contrast with th curves tool. Either of those methods will adjust contrast without altering the saturation of the image.

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