Basic problems & solutions

by Peter Rowlands

Reproduced from in focus 9 (Apr. 1985)

Underwater photography has been around long enough for all of the problems to be recognised and most of the solutions discovered. The result is that for every problem there is an equal and opposite solution even if a compromise is sometimes needed.

The problems start as soon as you want to take photographs underwater so the easiest solution is to change your mind and stick to land photography. This would make this lecture a lot simpler and you would be on far better terms with your bank manager.

However, there are those among us who like a challenge and hate saving money and it is thanks to them that most of the problems associated with underwater photography have been recognised and solved.

DENSITY

Almost all of the problems are caused by the medium in which we choose to operate. Water is many times denser than air and is capable of holding matter in suspension through which we have to take our underwater photographs. It is these two properties which cause the most headaches.

LOSS OF LIGHT

Water acts as a very efficient mirror and a sponge. Any light rays which hit the surface of the water at greater than 45 degrees are reflected back into the air.

The light which does penetrate is then absorbed far quicker than on land because water is much denser. The result is that light levels drop drastically the deeper.you go.

In addition to its density, water holds matter in suspension which scatters light around and hinders its path into the deeper water. This ability to suspend matter causes "visibility" problems. The poorer the visibility, the harder it is to obtain quality results.

The solution is to try and dive when the water is clear and calm as choppy seas reflect the light back much more. In addition, if you dive at midday, the light will be at its brightest and if you keep in shallow water there should be plenty of light.

LOSS OF COLOUR

The next problem is that water affects colour. The clearest purest water has a strong cyan or bluey/green cast and absorbs different colours at different rates. As a general rule, the result is that red is absorbed at approximately 15 feet, oranges at 30 feet, yellows at 60 feet and greens at 80 feet. This only leaves blue and black which doesn't leave much scope for colour photography.

The solution is to stay shallow where none of the colours are affected or if you wish to go deeper, take your own light with you to restore the colours. This is done by using flashguns or continuous lights. The colour cast can be reduced by using filters ? red to reduce the blueness associated with tropical water and magenta to reduce the greeny British waters.

LOSS OF CONTRAST

The next problem is that the visibility underwater reduces your horizons. 100 foot vis on land causes major aggravation but would be ideal for the underwater photographer. Not only does it limit your horizons it also reduces the contrast and this in turn affects the clarity of your shots. With black and white film the blacks are grey and the whites aren't clean whilst with colour film the colours are muted.

The solution is to keep the water between you and your subject to a minimum so that the contrast is as high as possible. Adding extra light will also increase the contrast but may cause further problems by lighting up suspended matter. Fortunately, modern films are much more contrasty than before.

REFRACTION

As if this is not enough, there are more problems. When light travels from air to water it slows down as it meets the denser medium.. This in itself is not too much of a problem if all light rays could cross the dividing line at right angles. It is when light travels from air to water at an oblique angle that we encounter refraction. This refraction causes light rays to bend and so affects the performance of a lens. The affect of refraction is most marked when using ultra wide-angle lenses behind flat ports. The normally square tiles in a swimming pool show a pincushion or pillow distortion. This distortion is most noticeable at the edges where sharpness also suffers.

Lenses behind flat ports such as the Nikonos 35 mm standard lens change in angle when they are used underwater. A 35 mm lens becomes roughly a 50 mm and a 24 mm becomes a 35 mm. For those of an optical bent, this has been greatly simplified.

The solution is to use a dome or correction port which aims to organise it so that the light rays travel from air to water at right angles. More complicated Ivanoff correctors are available on more expensive underwater lenses. These ports will rectify the distortion and if correctly positioned will restore the angle of the lens.

MAGNIFICATION

In addition, refraction causes a magnifying effect which makes subjects appear to be 1/3rd larger and therefore nearer. This only causes problems if you start to physically measure distances underwater. If you do wish to measure, one foot on land is equivalent to 10 inches underwater i.e. you have to move further back to make the subject look the same size as it would do on land.

So the solution is very simple - don't measure anything underwater. You are only concerned with how far away the subject appears to be.

FLOODING

Finally, water is always trying to cause as many problems as possible. As if its ability to bend light, reduce angles and absorb colours is not enough, it is also trying to get into your camera where it knows it is not welcome.
Later in the year we will discuss maintenance of equipment which should eliminate floods and other equipment problems.