What do Curves actually do?

The Curves are very like the Levels in that they are used to adjust the tonal contrast of the photograph. More importantly they are also used to adjust the look of the various grey tones of an image. The difference between Curves and Levels is that the Curves dialog allows you to control any portion of the grey tones whereas the Levels changes all of the various tones at the same time; you have less control with Levels than you do with Curves. This is why professionals prefer using Curves; they put you in control!

Use the Curves command to change overall contrast by:

1. Choosing the first tone in the image you want to have a pure black value.
Known as black point adjustment, moving the black slider along the bottom of the grid in the Curves dialog allows you to set which tone will be pure black in the photo. Bear in mind that if you move the slider past the start of the graph shape you will lose some of the subtle really dark tones and this will lose details. For example, if you move the slider to the right so it is at the very left end of the graph (the value of the tone at this point will be shown in the Input box), this value will be changed to pure black (value 0) and all the other grey tones will be darkened proportionally. If you move the slider even further to the right i.e. make a slightly higher tone value pure black, you will lose any tones shown in the graph that are to the left of the slider. Let's say the very end of the graph shows an Input value of 20, this is the darkest tone in the image and so when it becomes pure black we do not lose any other tone values because the other tones have higher values (they are lighter in tone). However, if we move the slider further into the graph, say to Input value 30, we will lose all of the tones between 20 and 30 because they will all be made to be pure black.
Now we know how to set the darkest tone of the image to pure black to obtain rich looking shadows.

2. Setting the pure white tone of the image.
Now we use white point adjustment to set the lightest tones of the image. Moving the white slider to the left so that it just touches the right end of the graph will make the tone at that position become pure white. Just as with the black point adjustment described above, since all the other tones below this point are lower in value (i.e. they are all darker), we do not lose any of the subtle light tones. However, if we move the white slider further to the left so that it is inside the graph shape, we will lose all the tones to the right of the slider as they will be made pure white in the image.
Setting the end points in the Curves dialog as explained above is exactly the same as setting them using the Levels dialog. For the end points the two commands do exactly the same job. The real power of Curves is in how we can control the other tones of the image as we shall now see.

3. Controlling the intermediate tones of the image.
The first difference you notice between Levels and Curves is that there is no grey slider shown in the Curves dialog and that there is a diagonal line from one corner to the other that cuts across the histogram shown behind this line. (To understand more about the graph in the Curves dialog see my article on understanding the histogram.) This line is actually a curve without any bends in it! When the line is straight (as it will be when you have only set the end points in 1 and 2 above), the relationship between the intermediate tones is being affected equally by the line. However, you can now click and drag anywhere on this line to create a curved line. When the line becomes a curve shape it no longer controls the tones equally. The real power of the Curves adjustment is that you can click and drag from several points on the line to create any curve shape you want. By changing the shape of the line into a curve you can control which portions of the tone scale are adjusted. You can also simply click on the line to create fixed points that act as anchors to stop the line from curving. When the line remains straight no changes occur to the tones in that area. This is how you can select just a portion of the tones to alter; create several anchor points along the line over the tones to preserve and then bend the line up or down in the area you do want to change.

A word of caution - if you have the curved part of the line that covers the shadows of the graph higher that the curve over the lighter tones you will cause the tones to reverse. This is known as posterisation and is not good! Try to make sure the curve is raising all of the way from left to right and that it doesn't dip down too much in the middle section.

There you have the 3 basics of using the Curves command. Using Curves is the most powerful way to control the contrast of your photograph and you should now be able to use it with confidence.

Photoshop for beginners - The 3 Basics of Using the Curves.

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