It has been a while since I posted, mainly because I've spent every free minute processing wedding photos. Now I've moved onto the honeymoon set and have been using this technique a lot on the underwater images.
Taking photographs underwater is great fun but due to the way light travels through water you get two very common problems: low contrast and a blue-green colour cast. These problems get progressively worse as you go deeper because longer wavelengths of light (red end of the spectrum) can't penetrate as deeply. The image below was taken in the Seychelles and illustrates the issue well. It was only a couple of meters down but already you can see the colour cast and the image looks really flat.
You can fix this by taking your own light source, e.g. a flash or a torch but that only really works to light small areas. A wide angle like this has to be post-processed. So what can we do about it? The usual Photoshop way is to bring up levels and move the sliders until this hit the edges of the histogram as so:
This gives an image with a bit more contrast but you can see it still has a strong blue cast.
Remember that the water effects different wavelengths of light by different amounts so should use the levels command to manipulate each channel independently.
This gives a far better result. Lots of contract and a full spectrum of colour.
Just remember, you can't put back what wasn't there so as you go deeper you will lose red colour completely (red objects look black at 25m or so). If you use this technique then you can end up with desaturated, almost black and white photographs as shown below.
