Lesson 2
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The Underwater Environment

• Light in the Sea
• Light levels
• Backscatter
• Magnification
• Contrast
• The Laws

Light in the sea

Unlike air, water is a very dense medium, and while your wineglass of Perrier water may look perfectly transparent (apart from the bubbles), water does absorb light quite quickly with depth.

Each colour in the white light spectrum is absorbed by filtration to a different degree by water

Figure 1. Light absorption with depth of water.

and it is essential that you understand that this applies equally to a horizontal body of water as well as a vertical body (or column). Absorption is proportional to the amount of water the light passes through. It is not related to the depth, or consequently the pressure of the water, but is cumulative.

Figure 2. Light gets similarly absorbed horizontally.

Much of the red colours will be lost if there is more than 5 metres of water in the path from the light source to the subject to the camera lens, and many of the other colours will be muted as well, as they lose their red components. Note that this will be 5 metres at midday, but less when the sun is at an angle.

So if you want bright colourful photographs then you must reduce this light path by:

• getting the light source close to the subject (to cut down absorbtion on the way)
• getting the camera close to the subject (to cut down absorbtion on the way back)

The stock solution to this problem in underwater photography is to

1. Use Underwater strobes/flashguns to provide a close white light source
2. Use Wide angle or macro lenses to get close to the subject

Of course, not all underwater photography is exclusively dominated by this technical solution, and alternatives are;

• Shoot in the top two metres of water below the surface - before colours have faded. The ripples in the water also cause varied and attractive lighting.
• Shoot silhouettes or wreck shapes - relying on tonal contrast rather than colour contrast to make your subject interesting.
• Shoot on monochrome film stock (no colour problems with black and white!)
• Use the new Kodak Underwater film - a slide film with extra red sensitivity - for use in 3-6 metres depth with natural light (or colour compensated flash).

Light levels

We now know that water absorbs light very quickly, even in seemingly clear water. In addition to this, not all of the sunlight that does fall on the water actually penetrates to the depths below.

Light entering the water at an angle near to 90 degrees to the water's surface, such as the midday sun, will enter easily, but light from more acute angles, such as sunlight in the early morning and late afternoon, or when the sea's surface is choppy, will tend to have a larger proportion reflected back into the sky than that which manages to penetrate underwater. Worse still, any particles or algae in the water will scatter the light in all direction, cutting down even more the light that manages to penetrate the depths.

The net effect is that below about 15m, even with a strong midday sun on a calm day in the tropics, available light is considerably reduced, and there is very little contrast, as the light source has also been further diffused and scattered by the water.

In reality, very little underwater photography is done with available light, the majority using a strobe in most circumstances to restore colour and contrast, but there will be occasions when you want to capture an image of an object, such as a wreck, which is too large to light artificially, and you must make the best of what light there is.

The actual light level is often hard to judge, as the human eye does a superlative job in adjusting for varying light levels, colour balances and contrasts, but you can be assured that the vivid colours and crisp shapes you saw on that 25m wreck will come out looking grey and muted in your photographs.

Backscatter

While strobe light solves the problem of colour loss, it introduces another problem, that of backscatter, which is where small particles (sand or organic material) or plankton, suspended in the water, reflect light back, like little mirrors, at the camera.

Backscatter can fortunately be minimised by

• Moving the strobe away from the central axis of the camera; this way less light is reflected into the camera lens
• Getting closer to the subject; less water in the way means less particles to reflect the light
• Not stirring up the seabed and compounding your problems with clouds of sand and mud
• Diving in the best visibility available

Magnification

Because light travels at marginally different speeds through different materials, it bends when it hits interfaces between unlike materials,

Figure 3. Magnification of objects under water.

Underwater, the interface will generally be between the water and flat glass, such as a diving mask, and this acts as a magnifying lens.

The relevant consequences of this are that:

• Objects will appear closer than they really are (by about one third). Where they appear to be is known as the 'virtual image', where they really are is known as the 'actual image'.
• When aiming strobes, especially those mounted to one side of the camera, it is easy, and a common mistake, to aim at the virtual image (what your eye sees) rather than the real subject. Consequently you are illuminating the water in front of the subject, rather than the subject itself. This will manifest itself as a bright right, and a dark left of the picture.
• Land lenses in housings must be capable of focusing on the virtual (closer) image. This may limit some lenses, or require special close-up diopter lenses to adjust. Wide angle land lenses (wider than 35mm) will probably need to be housed inside a dome port, which is discussed later on.

Luckily, focus markings on underwater lenses, such as the Nikonos 35mm, usually refer to the virtual image, allowing you to set your focus distance according to your normal judgement, rather than having to calculate the difference underwater.

Contrast

Contrast is most important in underwater images, and is easily lost. Frequently photographs taken using ambient light at the beginning or end of the day are lacking in contrast due to the lack of light, which can only be restored in the foreground. Backgrounds under such conditions are often rendered a rather muddy colour, and not the crisp blue that can normally be expected in clear water in the middle of the day.

If you get such results, don't blame your equipment - just dive at different times.

The Laws

You just can't get away from somebody's laws. In diving, there is Boyle's Law, Dalton's Law - the list goes on!

Underwater photography also has its laws, primarily Snell's Law, which describes the refraction of light when it passes the interface between two substances, and Beer's Law which describes absorption. This is all naturally a bit academic, unless you are designing dome ports or something similar, but there is one practical application.

At certain angles in the water, Snell's Law permits light to pass directly underwater, allowing the underwater photographer to see through and past the surface. This allows shots to be made, looking up, which can include details above the surface, such as people on a boat, or clouds in the sky. This circular, or elliptical area which can been seen through is popularly

This page was last updated on 11 August 1998
Please address any comments to Mark Mumford