Surface effects The shape of the water is decisive on how the light passes through it. Coming from an optically less dense medium (air) and entering a denser one (water), the light is partly reflected back while partly entering the water. Depending on the shape of the water, the light forms crinkle patterns or becomes diffused randomly in all directions.

The amount of light that is reflected upward depends strongly on the height of the sun (place on Earth, time of day and season) and the condition of the sea. A rough sea absorbs more light whereas a mirror-like sea reflects more. In the tropics, the sun stands straight overhead at mid-day, resulting in little loss. In temperate seas during winter, the light diminishes by as much as 3 f-stops immediately under the surface.

As a matter of interest, the reflected light is partly polarised (horizontally) and so is the part that enters the water (vertically). Polarisation is maximal in the early morning and late afternoon when the sun stands low in the sky. The vertically polarised light entering the water makes objects less shiny, more colourful, and can be used creatively, for instance to capture the deep colours of shiny fishes in natural light.
 
 
 

The diagram shows the theoretical loss of light due to reflection. The top left quarter shows sun rays reaching the water's surface. The top right quarter shows the amount of light reflected and the bottom right quarter that of light transmitted. The hours shown are not those of the clock but of the height of the sun. Only at angles less than 30º with the horizon ('four-o-clock') is the light reflectance significant and does loss of light become noticeable under water. However, in practice, and perhaps due to waves and the light diminishing towards sunset/dawn, the light under water diminishes much more quickly. At 'four-o-clock' one loses a complete f-stop (50%). Note how the light enters the sea at a steeper angle (blue lobe), which means that most of the time, the light comes from almost straight above, which limits natural lighting options. The light reluctantly enters a dive mask for instance or poorly lights subjects from their sides.

A sand flounder enjoying the extra camouflage afforded by the rapidly moving light patches, known as crinkled light. In an environment where everything moves, even its movements are no longer noticeable! This photo was taken with a warming filter and short time exposure, through a 50mm lens.

To increase the crinkle effect:

• Choose very calm water in sheltered spots.
• Stay close to the surface
• Use ambient light only. No flash light.
• Use short time exposures

• Choose an overcast day
• Choose ruffled water
• Work deeper down
• Use flash light to overpower ambient light
• Use long time exposures

Do as for the crinkle effect but also:

Choose a dark background such as a rock wall

Choose a dirty water patch with suspended particles

Shoot close to the edge of the light

Aim the camera towards the light

Avoid shooting directly into the sun. Hide the sun behind an object.

Shoot during a clear blue sky; avoid clouds

Use a wide angle lens

Deeply penetrating sun 'rays' are rare, so use your chance well!

Blue Maomao fish lazing in the mid-day sun in a narrow channel near Goat Island, in the Goat Island marine reserve, New Zealand. The example shows how the sun ray effect has been captured by positioning oneself near the edge of the light, close to a vertical wall and shooting towards the sun in a blue sky. A fill flash was used to bring colour to the foreground and to make fish visible in the shade. No colour correction was used in order to accentuate the blue colours of the fish. Note that some clouds are visible in the sky. This picture would have been better without them. Lens: 15mm

A snorkeldiver enjoys herself in a shallow alcove of Ngaio Rock, Poor Knights Islands, New Zealand. This photo was taken with a full daylight correction filter before the lens, shooting upward towards the sun but hiding it behind a steep rock face. This cove was carefully selected as an under water 'studio': a steep wall on both sides to make the sun rays stand out against a dark background; and some weeds in the dark lower right corner to frame the picture. The sky is entirely blue. No flash light was used. Lens: 15mm

The water's surface has further consequences for how light continues its path. The diagram shows how light is broken (diffracted) by the surface. A vertically incident ray passes without breaking but as the incident angle (height) becomes less, the light ray is bent to descend more steeply. Finally, light from the horizon passes at an angle of about 45 degrees or more precisely half of Snell's Angle, named after the Dutch astronomer Snell who discovered and described this effect (see separate box). To photograph the full circle from below, one needs a fish-eye lens of focal lens less than 12mm. A 15mm wide angle lens captures a good part of it.

Proper use of technique shows cathedral light and Snell's window. The sky is blue without a single cloud. It is 08:00 and the sun comes in at a low angle, the lake's water is perfectly still. The diver obscures the sun as sunrays radiate all around. 13mm fisheye.

A less perfect result as the water is ruffled, creating a bright patch around the canoe that hides the sun. But the subject is perfectly framed inside Snell's window. 13mm fisheye.

In the example, a mangrove tree has been photographed from below, offering a slightly distorted view of the world above. Snell's Circle runs through the middle of the frame and the horizon is the edge between light and dark. The bottom part of the picture reflects the bottom but about 2 f-stops darker. The photo would have been improved with a graduated grey filter correcting for one f-stop and by applying a half intensity fill flash to the branch in the foreground. At the time this photo was taken, these problems were not apparent.  On the right the same image corrected by techniques explained in the Digital Darkroom chapter. When shooting negative film, one can over expose by one f-stop, which gives one the opportunity of applying a grey filter in the Digital Darkroom.

The Dutch astronomer Willebrord van Roijen Snell (1580-1626) discovered the important law of light diffraction between two media having differing refractive indexes or optical densities or light speeds. The Dutch physicist Christiaan Huyghens later formulated other optical laws in his treatise on light. Snell formulated that light is refracted (bent) towards the optical axis perpendicular to the plane between the media when going from a less dense to a denser media so that {sin (a2)}/{sin (a1)}=n1 / n2 where a1=incident angle,  a2=refracted angle, n1=density of air (=1.00), n2=density of water (=1.33) As angle a1 approaches 90 degrees, angle a2 reaches its maximum beyond which total reflection occurs (going from water to air). This critical angle or Snells 'window' is just over 48 degrees to both sides of the vertical: {sin (a2)}/{sin(90º)}=1/1.33 sin(a2)=3/4 a2=arc sin(3/4)= 48.6º Huyghens suggested to look at light beams as travelling fronts of light, like soldiers marching in file can be considered as rows of soldiers. When a light front or row of soldiers meets a denser medium, it is slowed down, causing the front/row to bend and travel/march into a different direction. Note that winds meeting a land mass on an angle, behave similarly.

Scatter and diffusion The way light diffuses as it interacts with matter, depends on the size of the particles. For the ultra small water molecules, blue light is bounced off in all directions equally, while the rest of the light passes through normally. This diffusion was described by the physicist Rayleigh and explains why both the sky and the sea look blue. The diffused blue light appears to come from all directions, particularly deeper down and it has the effect of reducing  contrast while dominating the natural colours. Particles as large as phyto plankton but not visible to the naked eye (0.1 to 10 micrometre) act like mist particles, reflecting all colour components of the light back to where it came from. This effect makes driving in the mist an undertaking. This form of diffraction was first described by the physicist Tindall. It causes images to blur but it also offers creative opportunities both above and under water.

Finally, the snow effect that plagues under water photographers and which is called scatter, is light bounced off visible particles like zoo plankton organisms or even their shed moults. Such 'snow' or 'jelly' or 'snot' often collects close to the surface and should be avoided.

• To minimise ambient scatter:
• work deeper than 3m down
• shoot across the ambient light
• avoid ocean swell that stirs up sand and dust
• use long exposures (0.25-1 second). Particles that move will disappear from the image
• use continuous movie lights rather than strobes
• move cautiously, make no dust
• move against the current: your dust drifts away behind you
• dive alone: one diver creates less disturbance and has more patience.
• use wide angle lenses, and stay close to the subject

This orange finger sponge was photographed in a rather dusty environment using an electronic strobe, which accentuates environmental dust particles.

The same orange finger sponge, photographed with continuous movie light and a sufficiently long time exposure. Although the dust particles are still there, causing scatter, they do so while moving across the image, leaving no visible scatter trail. Current and wave action are needed to make them move.

Scatter from strobe light Strobelight scatter is caused by brightly lit small objects close to both the strobe light and the lens. As the particle is usually out of focus, it projects the scape of the aperture onto the film. It is a photographer's nightmare because it spoils the photo but more annoyingly because it is never visible when taking the photo. In the drawing the red numbers illustrate that at half the distance from strobe to subject, the light is four time stronger. At half that distance again, 16 times! It decreases quadratically with distance, thus increasing dramatically towards to the strobe.

• Move the strobe further out. This requires a specially long arm or a buddy. By enlarging the distance, the lighting is no longer frontal but strongly sideways. A larger distance changes the quality of the light and makes it more blue. Note that the further out your subject is and the larger, the further the strobe arm should be extended. It is not unusual to have a 1.5m strobe pole for lighting models and parts of shipwrecks!
• Move the strobe away from the corners of the image, towards the centre of the long side. This effect is dramatic, but it also produces unnatural looking light. Also when turning the camera for a vertical shot, the side lighting becomes unnatural. But a compromise position can be chosen. Many photographers use the centre position for macro photography, as the light needs to be brought close in for subjects close to the lens.
• By using a wider aperture, the intensity of the aperture projection becomes less and it may fade into the background intensity. But your depth of field also becomes less. Below is an example of an impossible situation in 'snowy' water where a macro lens of 50mm was used to capture a fleeting fur seal. Aperture was set at f5.6 which resulted in a shutter speed of 1/125s. The combined effect of the bright background and wide aperture removed most scatter.

Tip: make yourself familiar shooting with a macro lens at f-4 and over one metre distance. You'll be surprised, especially in dark places!
• Arrange a lighter background, particularly in the quarter of the image where the flashlight comes from. This can often be done. Even the sea can serve as such a background provided that ambient light is sufficiently allowed to expose the film.
• Use a wide flash source. In the old days when flash bulbs were used, it was not unusual to have a flash reflector of up to 30cm diameter, which produced a softer kind of lighting. Nowadays the size of the strobe is very small, resembling a point source. Unfortunately, it is technically difficult to make the size of the light source larger. A large screen or reflector is also rather cumbersome under water.
• Blinker the flash beam by placing a blinker or baffle between the flash and the lens. This method is cumbersome and won't work well.
• Use more available light. By using the filtering techniques described later, the amount of flash light can be reduced by half, allowing for less strobe light and more ambient light. This is both easy to do and dramatic in result.
• Use continuous light instead of strobe light and longer shutter speeds of between 0.1 and 1 seconds. The moving particles will then not be recorded. This method has given stunning results in estuaries with less than 1m visibility but it requires a steady tripod, still subjects, a movie light and some experience.

• Fast recharging: the faaster the better, so you can make repetitive shots. 1-4 seconds
• Small size and light weight: this is a compromise. Obviously a powerful strobe must have bigger batteries and a bigger capacitor and a bigger light bulb, so it comes in a bigger housing.
• Reliable and rugged: the strobe will always be more rugged than your camera.
• Through-The-Lens exposure control: The camera controls the strobe. Very important for improving your success rate. Macro photography depends on it entirely. Take extreme care when mixing brands. A Nikon camera may not work well with a non-Nikon strobe. Modern cameras have very sophisticated flash strategies.
• Inbuilt slave circuit: so that the strobe will trigger even though not connected to the camera. This enables you to take photos with two or more strobes, and the strobe can be held by the buddy or model.
• Inbuilt exposure control ‘zoom’: when used on its own in slave mode, the strobe must be able to control its own output depending on the amount of light returned from the subject. Camera and remote strobe must agree on film speed and aperture setting.
• At least guide number 30 or 60: the guide number is standardised at a film speed of 100 ASA and distance in metres. Thus GN 30 means f11 at 3m, f16 at 2m, f32 at 1m. Note however that guide numbers are often slightly exaggerated and that underwater in temperate seas much light is absorbed by plants.
• 4 energy settings: energy settings are not so important anymore because they belong to the era of guessing strobe strength and setting it manually. However, the inbuilt 'zoom' function mentioned above, MUST have several energy settings, like f4, f5.6, f8, f11, f16 for mixed-light situations.
• Inbuilt search light: An inbuilt search light can often be placed central to the round flash tube, giving precise direction. However, this light runs from the strobe batteries and may consume most of their energy. Make sure this light is of the latest type, preferably with thrifty LED bulbs.
• External filter holder: the use of filters in underwater photography is not common, and you won't find a strobe with such feature. Read the tips and tricks chapter to make your own.
• Barn doors:  barn doors are external flaps to narrow the beam and although common in the studio, are seldom used underwater.
• Large reflector: old flash bulb strobes had large reflectors, which produced soft shadows and a threedimenional look. Present-day strobes are housed in cylindrical housings that are as bulky as their reflectors. Large reflectors have disappeared and it is difficult to make your own.
• Wide angle: if you have a wide angle lens, you must have a wide angle strobe. Be suspicious of diffusers because these do not (cannot!) produce even lighting. Yet for macro photography you may need another, smaller strobe.
• Plug and unplug in the water: in the old days it was possible to plug and unplug strobes in the water because the camera presented only a make-contact without voltages. Present-day strobes, however, present a number of signals to and from the strobe, often at high voltages and impedance. A single droplet of salt water can now seriously upset the reliability of your strobe. It is not a desirable situation, though.

f035222: a mother fur seal photographed under impossible conditions in a 'snowy' sea with less than 10m visibility and a 50mm macro lens. By using a wide aperture F5.6, the background became bright enough to hide most scatter. Also the shutter speed became fast enough. Note how the seal's body fades away, not distracting from the seal's eyes.

f034008: rock 'bommy' with varied seaweeds amongst sand ripples, photographed with a fisheye 13mm lens. The strobe was used to bring colour. No colour filter was used because of the short distance to the subject. Scatter disappeared because the strobe arm was long and bent backward, while also the background was kept light. f5.6 1/125s.

Loss of colour Water particles interact with light by absorbing certain wave lengths (see diagram). First the reds and oranges disappear, later the yellows, greens and purples and last the blue. Loss of the colour red is dramatic and is already noticeable at 50cm! At 5 metres depth some 90% has disappeared.  Since the loss of colour varies critically with distance, it is necessary to make corrections by applying colour correction filters. Their use is described later on and their effect is quite substantial. Note that colour filters applied under water, do not taint the blue background, but when they are applied in the computer or in the laboratory, they do. The picture shows that it is the total light path that matters. In the case of the strobe, this amounts to about twice the subject distance whereas for ambient light it amounts to depth plus subject distance.

The diagram on right gives accurate light extinction figures for one metre water. Where clear oceanic water (probably with visibility 50m) gives 40% loss in red light per metre (blue curve), the light remaining (transmission) is thus 60%. For two metres it would be 60% of 60% remaining, = 36%. Clear oceanic water has its least loss in the blue colour, which means that distanct objects would look blue. As a rule of thumb remember that you lose one f-stop (50%) in the reds for one metre light path. So a subject at 50cm distance from the strobe already loses that one f-stop. Average coastal water extinction in the reds (green curve) is about 70%, thus transmission is 30%. Over 2 metres the remaining red light is 30% x 30% = 9%. The dip of this curve rests in the green wavelengths, which makes distant objects look green.

Loss of intensity This graph shows actual measurements of light intensity (in f-stops) as it decreases with depth (in metres). The measurements were done in northern New Zealand towards the end of February, our summer in the southern hemisphere. Although well on its return to winter, the sun still stands high in the sky, causing minimal loss of light directly under the surface: just under two f-stops. In the tropics only one f-stop is lost, which can be traded off for finer grained film. In temperate seas it is common sense to use faster film like 400ASA. In mid-winter when the sun stands low in the sky, or in early morning or evening, the loss of light can be dramatic, amounting to 4 f-stops! The two measurements demonstrate the dramatic loss of light due to poor visibility (red squares), compared to good visibility (blue triangles). At 30m depth in poor visibility, almost no natural light photography can be done on moving objects. Yet, using a tripod combined with fill flash, remarkable results can be obtained, even at -9 f-stops with shutter speeds of 1 second or longer!

camera angles How to aim your camera is of critical importance underwater. The diagram serves to give you a feel of what the consequences are of your position in the water and the way you aim your camera. Read it with care. a The camera faces down, close to the surface. Although this is a most convenient way of taking photos, you have serious problems: Close to the surface the plankton skeletons and dead animals collect, and there are bubbles from waves - a rich source of scatter. You have the light coming from right behind and this is the worst angle to show up scatter from all three sources: Rayleigh (molecules), mist and snow. Frontal lighting of the underwater world does not show its form. There is also little contrast. The light path from camera to subject to surface is maximal, thus everything looks at its worst - blue You are furthest away from your subject, causing unsharpness You are in the place where the water moves most, inviting for motion blur.

• b This is an acceptable position and angle. You are close to your subject which is side-lit for maximal contrast and shape recognition. The light path from camera to surface is shortest. Your subject is positioned against a dark background. But the dark background may accentuate scatter from the strobe.
• c Your camera is looking up towards the light, and although you gain from the benefits of position b, you'll have some problems.
• The top of your picture looks very bright and the bottom very dark. There's so much contrast that it cannot be reproduced on film or print. The water looks white.
• You look at the dark side of your subject. There is no colour or registration of detail. But this can be an ideal position for photos of jellyfish, as long as the background is evenly dark. Don't use a strobe.
• d The subject has been placed between camera and light source, which reduces most of the light and contrast problems. You may see an aura of cathedral light shining around your subject, particularly when using a wide angle lens. Because you are looking towards the light, scatter is minimal. Animals with translucent parts (jellyfish, fins, eyes) may cause spectacular images. In deeper water Snell's circle may show while also the water has more colour. But you may have problems.
• The subject is silhouetted black against a very bright background, showing neither colour nor detail, but a fill flash can fix this while causing the least scatter.
• Water ripples may spoil the effect, creating a large white background.
• e Has fewer disadvantages than c since the camera looks into a bland bright space without a large contrast between light and dark. Always try this possibility. A fill flash is needed for opaque subjects since their sides are poorly lit. Because of the light background, scatter is minimal.
• f The camera now looks more at the distant blue sea, while the subject is lit from its side. Such images give stark graphic effects with blue backgrounds.
• g This is the position and angle to capture cathedral light, particularly when a wide angle lens is used. Notice that the camera hides just in the shade and that the subject must appear in the light or twilight zone. Move around and study the effect as your position changes.
• h You are looking through a shaded patch of water towards the subject which appears in the light. The water appears unusually clear because there is no scatter from water molecules or particles. Your image becomes sharper too. With a bit of luck the rock face may form a natural frame for your subject. Look for this effect, which depends also on the type of lens used. A 35mm (!) lens can achieve surprisingly pleasing results. The following problems may occur:
• Using the strobe will spoil the effect, as it introduces scatter projected against the dark parts of the natural frame. Move the strobe to the side with most light.
• Your presence inside a cave will stir up copious amounts of dust. Take all the time to avoid this.
• Your bubbles will race along the ceiling, causing particles to rain down. You may have to hold your breath for a long time.

• There’s much more light: so you can use a finer film and have more depth of field
• There’s much more visibility: the water looks deep blue; the subject is always sharp, up to 10m away; there’s no scatter to ruin your picture. People love to be in these places and they love such pictures. So they sell well.
• The situation is dependable: tropical seas rarely have plankton blooms; the weather is better; the reefs are sheltered; seasons are weak.
• The sea creatures are more colourful: it makes your pictures just so much nicer; there are also more interesting creatures.
• The diving is pleasant: no more wetsuit or weightbelt; the water is so agreeable; the sun shines most times. You can work longer hours in the water. Time is money.