Wedding photographers love Curves

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And if you're a digital photographer so should you, since it gives you complete control over the contrast in your images.

Here's a shot of my chum Jack, taken last week, with the corresponding 'Curves' dialogue box in Photoshop. In light grey within the graph is the RGB histogram. To make it simpler to understand what curves can do, let's discard the colour information from the shot by converting it to greyscale.

The horizontal (X) axis runs from 0 (black) to 255 (white), encompassing all shades of grey. It corresponds to the 256 tones being input into the 'Curves' process. The histogram shows the relative number of pixels at all 256 of these tones. The majority are towards the darker end of the scale - the image can be described as 'low key'. The vertical (Y) axis is identical to the horizontal axis except that it represents the tones being output from the 'Curves' process. At the moment our 'Curve' is the line indicated in red. Choose any point on the X-axis, the input tone, travel vertically until reaching the red line or 'Curve', and then travel horizontally left to the Y-axis to see the output tone. At the moment 0 maps to 0, 255 to 255, 127 to 127 etc - the image remains unchanged.

Let's introduce some extreme contrast! The red path now shows the 'Curve' that I've input - look at the affect this has on the image. All input tones from 0 to 130 have been mapped to an output of 0 (black) and all input tones from 131 to 255 have been mapped to an output of 255 (white). The resultant image now contains only black and white - the ultimate in contrast but not so useful as a treatment for our images.

A more pleasing result is seen with the path in red shown above - known as an 'S-curve'. Darker tones have been darkened slightly and lighter tones lightened. The slope of the line in the midtone region has increased leading to a more subtle increase in contrast.

The same process can be applied to colour images but the extreme contrast procedure would make your image look rather odd (which is why I converted to greyscale). In Photoshop curves can be applied to colour channels independently giving great control over colour balance.

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