Optical Differences

Light absorbtion/reflection

Water is much denser than air and so light is absorbed much quicker than on land. In addition light is reflected back from the surface before it can get underwater so light levels are greatly reduced.

The solution is to dive as shallow as possible when the sun is at its highest and keep the distance from you to your subject to a minimum i.e. reduce the water path to a minimum.

Remember

LIGHT PATH = DIVING DEPTH + CAMERA TO SUBJECT DISTANCE

If you need to go deeper, consider a faster film speed.

Suspended matter

Water can also hold matter in suspension which reduces water clarity and so limits our vision underwater and reduces contrast. In addition this suspended matter reduces the light from the surface by reflecting it back.

By far the most important effect is the reduction in visibility. It is worth bearing in mind that if visibility

on land were to be reduced to just 30 metres many activities would grind to a halt yet we would consider this to be excellent visibility.

Once again keep your water path to a minimum by using a wider angle lens and a slower speed film will give more contrasty results.

Colour absorbtion

Water acts as a strong colour filter and even the clearest water contains a heavy cyan (bluish) cast. The more water there is, the stronger the cast. Where there is algae in the water this will alter the colour cast from cyan/blue to green such as in UK waters.

Filters can counteract the cast to a limited extent. A filter in front of the lens of the opposite colour to the cast will help remove it but the deeper you go the stronger the filter will have to be.

Colour is expressed in UNITS

OF STRENGTH and for every foot of light path 4 units of colour will be needed to eliminate the cast. Because a filter reduces the amount of light reaching the film, the exposure must be compensated. In general terms, for every 30 units of colour you will need to give one stop more exposure.

For example, you are in the Caribbean taking shots in 10 feet of water of a subject 5 feet away: LIGHT PATH =10 + 5=15 FILTER = 15x4 = 60 EXPOSURE COMPENSATION = 60/30 = 2 stops

It doesn't take a genius to realise that filters have a limited effect in that the deeper you go the filter will have to be so strong that exposure times become impractical. However, if you need to photograph at depth (to photograph flourescing coral for example) filter accordingly and, if necessary, be prepared to use a tripod.

Fortunately modern digital cameras have circuits which can adjust colour balance but, in most situations, they still need additional filters to achieve the correct result.

Refraction

Optically, water slows down light rays as they pass from air to water and vice versa. The effect of this is a deflection of the light path in certain circumstances and this will affect

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Refraction

The light rays are refracted (deflected) at the air/water interface. The greater the angle of approach, the greater the refraction. Light rays (2) are refracted more than rays (3). Light ray (1) passes through the flat port at right angles so is not refracted, just slowed down

With a dome port, light rays are undeviated as they all pass the air/water interface at right angles. The result is a restoration of the angle of coverage of the lens

Dome ports are a simple and cost effective way of correcting most of the problems of refraction but can cause flare as light rays bounce from the inner surface of the dome and into the lens. This is internal reflection and becomes more of a problem the steeper the curve of the dome. The solution is to mask the surface of the dome not seen by the lens or use a lower profile dome

picture quality. This problem occurs when photographing behind a flat port but only becomes a real problem with wide angle lenses from 28mm and wider.

The simplest solution is to use an optical port such as a dome which attempts to keep the light waves at the air/water interface traveling in a straight line. This way, even though the light is slowed down it is not refracted. The dome is a simple and sometimes flawed method of correcting refraction and the ideal is to design a lens purely for use underwater. This is more expensive but will give better results (if the design is good!)

Peter Rowlands [email protected]

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Book reviews

Out of the blue by Colin Bateman

This 180 page book is a comprehensive guide to the basics of underwater photography which is competently illustrated with informative captions and detailed text.

The unfortunate point is that this book comes at a time when underwater photography is hurtling into the digital age and I suspect that the majority of people starting to take pictures underwater will do so with a digital camera. The basics of photography do not change for digital but the way images are recorded varies dramatically and there is a new learning curve to follow.

This book will undoubtedly help a film based underwater photographer to improve their expertise and knowledge base but for those in or about to enter the digital age there will be other books which will cater for your needs.

www.paulkayphotography.co.uk

Colin Bateman died in 2002 aged just 49. His wife Lorraine decided to produce a limited edition 157 page book of his photographs to help generate funds for the ScubaTrust, a charitable organisation whose aim is to give people with physical disabilities an opportunity to experience the pleasure and excitement of snorkelling and scubadiving.

"Out of the blue" shows that Colin was a really gifted underwater photographer and in just 12 short years he took images of a very high standard.

For copies e mail Lorraine Bateman [email protected] or contact Ocean Optics e mail [email protected]

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