Filtered for Colour

Anyone who has taken still pictures by available light underwater will almost certainly have been disappointed with the results. This is because during the dive your eye adjusts to the strong blue/cyan cast in tropical waters or green in temperate seas. Unfortunately your film doesn’t and this results in your pictures being much bluer or greener than you remember.

The solution is simple but requires you to make a few basic calculations to get it right. The formula is that, for every foot of light path (this is the total of the depth you are at, plus the distance from you to your subject), you have to add four units of the opposite colour. For blue/cyan water this would be red and for green it would be magenta.

Colour-correcting filters are available in three main forms – gelatin, glass or resin. The former are easily cut into circles and attached to the front of your lens and held in place with a neutral filter such as a UV or skylight. The latter are more rigid so are more difficult to cut but they are more robust and can be cleaned with a soft cloth. Gelatin filters mark easily and cannot be used in water so are limited to use in housings. Glass filters are not so readily available but UR Pro in the US produce filters especially for underwater use (web site http://www.naper.com/urpro). They are more expensive but worth it.

The strength of the filter is designated numerically – the higher the number, the stronger the filter. The maximum colour is usually 50 units and CC50 Red will reduce light transmission by one stop so you will have to compensate for this unless you have a through-the-lens metering camera, in which case it will do it for you.

It won’t take a mathematical genius to work out that a CC50 filter will correct the colours for a light path of just over 12 feet (50 divided by 4) so you are limited to specific depth/distance combinations. But if you want your shots to come out naturally, it’s worth the extra effort of using a colour-correcting filter. They are especially useful for getting pleasing skin tones of snorkellers. A final word of warning though, is that if you use these filters at a shorter light path, your shots will have an unnatural red/magenta cast to them, so you must know your subject distances and filter up accordingly before diving.

It may sound a bit complicated and fiddly but filters are well worth it for available light shots. The photograph above was taken with a Nikonos V, 15mm lens, UR Pro filter and Fuji MS at 800 asa.

 


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