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Ocean Optics

13 Northumberland Avenue, London WC2N 5AQ Tel 44 (0)20 7930 8408 Fax 44 (0)20 7839 6148 E mail [email protected] www.oceanoptics.co.uk

Filter fulfilment 2

By Alexander Mustard

Back in UWP 11 Peter Rowlands wrote "Filter Fulfilment" an article encouraging us to use coloured filters as the alternative approach to using flash to get colourful underwater images. My aim in this follow up article is not to convince you that this technique works with before and after shots; PR's article convinced me! Here, I plan to go into a bit more detail and pass on some of my ideas about how to get the best out of this technique of combining coloured filters with a digital camera.

Colour compensating filters have been used in underwater still photography for many decades. Seek out a 1960s text and you will find lots of filter facts, but reading between the lines and it appears that filters were popular only because flash photography was, at best, temperamental and at worst, darn dangerous! Even the most experienced users of filters would struggle to get the exact colours they wanted - colours that could be so easily achieved with flash. Once electronic strobes got through their teething problems the popularity of filters waned.

The theory

A colour compensating (CC) filter is used underwater to attenuate undesired wavelengths (read colours) and to transmit desired wavelengths of light to counteract the filtering effect of the water. Correct filtration balances the full spectrum of

Digital cameras make getting colourful shots without flash very simple. Nikon D100, Subal housing. 16mm lens. 40CC Red filter. 1/ 100th @ f9.5

wavelengths, the wavelengths we would usually supply with our flashgun. It is important to remember that filters, either in the form of seawater or the one on your lens, work by subtraction, they can only take away unwanted wavelengths. So in filter photography we are always reducing the light that is available.

As an example, in clear water red, yellow and orange light is attenuated and the resulting spectrum is biased to cyan/blue. To counteract this we must add a red filter that will get rid of all that unwanted cyan and blue and flatten the spectrum (although at a much lower intensity that the original light). The reason this technique is never particularly effective on film is that the exact filter required to "correct" the spectrum depends on what light has been removed by the water,

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