For this tutorial I'm using GIMP, the free open-source image editing program, but I will also be referring to Adobe Photoshop Elements 8, and the techniques used in this tutorial will also work in all recent versions of Adobe Photoshop, as well as Corel Paint Shop Pro, PhotoImpact and other advanced image editing programs.
If you've ever taken the time to explore all the options and menus of your digital image editing program, you've probably come across the Curves function and wondered exactly what it does. In GIMP you'll find the Curves function in the Colours menu. In Photoshop Elements 8 you'll find it in the Enhance menu under Adjust Colour.
The Curves dialogue is a daunting looking thing, looking like a graph with a diagonal line running up the middle of it, like a difficult piece of maths homework. It's nothing to be scared of though, and is in fact one of the most useful and versatile tools for making adjustments to the exposure and tonal balance of your digital photographs.
The Curves tool is indeed a type of graph, specifically a histogram, a graph showing the relative strengths of different tones within the image. Many digital cameras have the option to display a histogram while shooting, which is very helpful when setting exposure in tricky lighting conditions. The histogram shown in the Curves tool is exactly the same; the graph shows the input distribution of tones along the bottom of the graph, and the output tones along the vertical axis. The straight diagonal line from corner to corner indicates that there is a 1:1 ratio between input and output at all tones; the output is exactly the same as the input, as you'd expect. It is by adjusting that line, and altering the ratio of input tones to output tones, that we can achieve a number of effects. By general convention, the left-hand end of the horizontal axis represents the darker tones, while the right-hand end represents the highlights.