A recent discussion of ambient light underwater photography in the Wetpixel forum pages presented a wealth of information on the use of filters to correct color balance problems in photographs taken underwater without strobes.

But filters cannot fix color problems when shooting in ambient light at depths where red and yellow wavelengths of light are completely absent, not merely attenuated. Alternatively, the subject may be too large for conventionally mounted strobes, or the photographer working with strobes at depth may be unable to approach the subject close enough to accommodate the limited illumination range of strobe lights underwater. In either case, the result is that the subject is illuminated solely by ambient light in which red and yellow wavelengths may be deficient or totally absent.

The image recorded electronically or on film often looks even worse than our subjective recollection of the actual scene. This is because the human optical system is very good at adjusting to color casts that dominate the visual environment. When our eyes are flooded with a light of a certain color, the visual system compensates by becoming less sensitive to that wavelength.

The camera simply records what it sees and reproduces the wavelengths of light that enter the lens as faithfully as the design of the film emulsion or CCD sensor allows. When the printed image is viewed under normal illumination or an electronic image is displayed on a calibrated monitor, however, our visual system no longer compensates for the prevailing color cast, so the image exhibits a much heavier blue and/or cyan color cast than expected.

Examples can be found in the pages of almost any dive magazine. Images of cetaceans, sharks and many large pelagics taken in open water are classic cases. For really bad color casts, flip through any shark ID book. But you don't necessarily need to throw away that once-in-a-life time shot. Adobe Photoshop contains an array of powerful tools for correcting color casts. This tutorial introduces one of the most useful techniques for dealing with extreme problems, known as channel mixing, or