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| Click photo for more images from the book "Skin." |
The primary tool for color and tone manipulation in Photoshop is the Curves dialog. You open the Curves dialog either by choosing Image > Adjustments > Curves or by making a new Curves adjustment layer. Using an adjustment layer is the better method: Click the Adjustment Layer icon at the bottom of the Layers palette and select Curves from the resulting menu.
The first time you enter the Curves dialog, you'll see the standard, straight diagonal line graph. This is an xy plot of input to output values -- input values plot along the bottom and output values plot along the left edge. The diagonal line represents the relationship between input and output. This perfect diagonal means that there is no change (that is, x equals y), and output values are the same as input values. The gradient along the bottom and the left edge indicates the progression of values from light to dark as they are mapped to the line.
Sometimes the gradient is set up to represent ink densities so that as it progresses from left to right, it gets darker -- adding density. Photographers working with additive color, as with RGB, usually prefer to think in terms of adding light so you can reverse the gradient by clicking anywhere inside the gradient.
You can use a finer grid by Option/Alt+clicking the grid to toggle between fine and coarse views. The input and output values for any point that the cursor is over will be shown below the curve.
The Channel drop-down at the top of the dialog allows you to affect all channels or individual channels depending on what is selected. We will confine ourselves here to examining the contrast and tonal effects of the curve so we will only look at the composite (RGB) channel.
Contrast Control: The Basic Curve Shapes
In several of the photographic figures accompanying this article, different curves are applied to a black-and-white image, and a 10-step Zone scale is placed at the bottom of the curve dialog in each to help you visualize the different effects of various curve shapes. (Click the image at top to scroll through the slide show and see the different effects of different curve settings.)
Keep Saturation in Mind
So far we've been looking at monochrome images because we've been concentrating on tone and contrast. When we start including color, we have to be careful with contrast-enhancing curves because they almost always increase saturation as well. If you need to increase contrast without increasing saturation, you can use RGB curves in an adjustment layer and change the Layer Apply mode to Luminosity.
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