Curves

This tool is similar to levels it that it provides a more exact way of improving the contrast of an image. However, whilst the levels tool allows you to adjust the upper and lower bounds of the image luminance and its midpoint, the curves tool allows you to manipulate the whole spectrum, boosting or attenuating any luminance band in the image. In later versions of Photoshop it is possible to convert from a levels adjustment to an equivalent curves adjustment, and this can sometimes provide a good starting point for a subsequent curves adjustment.

Again, it is very useful to have a good understanding of how luminace histograms work.

A Practical Example

In Photoshop, the curves tool is on the Image>Adjustments menu. In the GIMP this tool can be found on the Tools>Colour Tools menu and also directly on the Colours menu.

Conference hallLet's use the same image as we used for the levels example. Because of the hazy, diffuse light that day, the image is a little flat and lacking in dynamic range.

Levelled imageBy adjusting the curve we can increase the dynamic range and give the image more impact.

It is very easy to overdo the curves tool, so be careful not to make the adjustments too large. The example on the left is probably over done, although the contrast in the building and the courtyard paving is certainly stronger.

Curves DialogAdjusting the Curve

The curves dialog for this image is shown on the right.

The bottom left point of the curve represents black, the lower portion of the curve represents shadows, the middle of the curve represents midtone, the upper portion of the curve represents highlights and the top right point of the curve represents white.

When the curve is a stright line going from the bottom left to the top right, no adjustment is being made. This is the default setting for the tool. You can add points to the curve by clicking on it. These show up as little boxes. Those boxes can then be dragged to adjust the shape of the curve.

Experiment with different settings curve adjustments and see how this affects the image.

You can find a good tutorial on the Photoshop curves tool at the Cambridge in Colour site.

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