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A couple of people have asked me for tips on taking photographs underwater, and there are always questions about it on the scuba newsgroups. Underwater photographs often have a blue or green cast. I've seen several references to "The Mandrake Technique" for colour correction but I've yet to find a definition of what this actually is. I'm going to describe what works well for me.

I use a compact digital camera with built-in flash. It doesn't matter whether you are in the UK or the Red Sea, getting closer helps reduce back-scatter from the flash. This might mean getting within a couple of metres in clear water or a matter of centimetres on fairly typical UK dives. If you can't get close enough to eliminate back-scatter, or feel that flash will upset your subject, you have to settle for taking the photo by available light and temporarily putting up with the colour cast. I say 'temporarily' because some photos can be post-processed to remove or at least reduce the cast.

Here is a picture of a pike in Stoney Cove. It has the green cast that is fairly typical of a UK dive.

Using Paint Shop Pro to split the picture into its red, green, and blue components, you can see that there is still quite a bit of detail in all three components, so we should be able to produce a colour-corrected version.

Using the histogram adjustment tool, select the red channel. You can see that the range of red intensities is quite narrow. Ideally it should pretty much cover the fully width of the graph. We can stretch the red component by moving in the 'Low' and 'High' indicators just below the graph, to just cover the range that is present. Just drag the little triangular indicators.

Do the same for the green and blue channels.

The result is a little too red.

I can reduce the red content either by opening up the range a little at the top end, effectively pushing the red content towards the darker end of the histogram,

or by reducing the output brightness using the 'Output max.' slider on the left edge of the graph.

Either way the result is pretty much the same. Here is the final result alongside the original. Not bad for a process that takes seconds and requires little or no thought.

After	Before

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Page last updated 15 June, 2007