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Underwater flash


Off-camera flash reduces or eliminates backscatter, providing a clear image. Photo by Tom Sheldon.
Off-camera flash reduces or eliminates backscatter, providing a clear image. Photo by Tom Sheldon.

Although continuous light and flash bulbs can be used for underwater photography, electronic flash is the artificial light choice of most underwater photographers. Its light is balanced for daylight-type film and its brief flash duration reduces blur caused by camera or subject motion.

Because of the light absorption characteristic of water and a phenomenon known as “scatter” or “backscatter,” much of the light from electronic flash is lost underwater.

WHAT IS “BACKSCATTER?”

Lakes, rivers, the ocean - indeed all natural bodies of water - contain tiny particles in suspension. These minute pieces of matter can be just about anything - sand, plankton, vegetation and even air bubbles. Although they are present in even the clearest of natural water, large concentrations of them make water murky. The light from flash reflects and deflects off the small surfaces of these suspended particles, causing the light to diffuse and scatter. At its most severe, when the diffused light partially or totally obscures the subject, it is called backscatter.

A similar effect occurs on land in fog or smoke. Shine a bright light at a subject in fog and the light becomes scattered by the particles of moisture. If the fog is particularly dense, subjects that you might otherwise see can be totally obscured by the light scatter.

Although refraction may prevent full illumination of the subject, careful framing ensures that the center of interest will be properly lit. Photo by Tom Sheldon.
Although refraction may prevent full illumination of the subject, careful framing ensures that the center of interest will be properly lit. Photo by Tom Sheldon.

REDUCING BACKSCATTER

Underwater backscatter is at its worst when using on-camera flash, where the flash head and lens are located close to each other, since much of the light gets reflected almost straight back to the lens. The effects - whether underwater or in fog or a snowstorm on land - are greatly reduced if you move the flash head away from the lens (off-camera) and place it higher so that is angled at 30 to 45 (in relation to an imaginary line between camera and subject) to illuminate the subject. The ideal set-up is a flash mounted on a bracket that is connected to your underwater camera or mounted on adjustable arms that attach to the camera's watertight housing.

Positioning an off-camera flash to one side and aiming it at the subject has the advantage of showing texture and three-dimensionality. Sidelighting generally provides more visual interest to an underwater image than flat, frontal lighting.

If the flash produces light with a sharply defined edge between its light and the ambient light (which could be total darkness at great depths), there is less risk of illuminating particles that are outside of the main beam of the flash, and scatter is further reduced.

In really murky water, we've heard of photographers who bring a large, clear plastic bag with them underwater - the bag being filled to capacity with distilled water that is completely free of particles in suspension. The bag of clear water is placed between the lens and the close-up subject to displace the murky water and thereby obtain a clearer picture.

Sidelighting reveals the texture of this colorful leather star as it engulfs a scallop. Photo by Tom Sheldon.
Sidelighting reveals the texture of this colorful leather star as it engulfs a scallop. Photo by Tom Sheldon.

LIGHT ABSORPTION

No matter how the flash is positioned, scatter still accounts for a reduction in the amount of light that strikes the subject. The murkier the water, the less light gets through. Remember though, that even the clearest water with no apparent murkiness will absorb light, with the result that twelve feet is about the maximum range for underwater flash. The range is less with smaller flash units.

A flash guide number is useful when calculating the correct aperture setting for photography on land where air is the medium that the light travels through. Underwater, you have to divide the flash’s normal guide number by at least four to make up for the loss of light. Experience will be your guide in deciding if further reduction is necessary based on the degree of murkiness of the water.

FOCUSING IN REDUCED LIGHT

When using flash, you may require a battery-operated tungsten light to enable you to focus and compose an image in deep water where there is little natural light, or if you are diving at night. Some underwater flash units come equipped with an "aiming" light that performs the same function. Like a modeling light in a studio, the flash’s greater intensity will usually over-ride the relatively-weaker light from the focusing light, permitting you to keep the focusing light trained on the subject even when the flash exposure is made.

REFRACTION

Refraction not only affects the apparent distance differential between subject and lens, but also affects the angle of a flash unit’s output, reducing it by about 40 percent.

A standard electronic flash unit provides coverage of about 70 on land, and around 40 underwater.

If a moderately wide-angle lens has a typical field of coverage (angle of view) of around 70, then there is a risk that the flash unit will not fully-illuminate the subject, and care must be taken to ensure that the flash illumination will cover at least the picture’s center of interest. Locating and aiming the flash unit in relation to the angle of view of the lens and the flash unit’s output becomes a balancing act to ensure that there is uniform lighting on the subject.

Refraction reduces flash angle by about 40%.
Refraction reduces flash angle by about 40%.

CAMERA TO FLASH COMMUNICATION

In all but integrated, specialized underwater camera systems that are used without a housing, the camera in its housing is connected to the electronic flash unit (in its own underwater housing) via a synch or PC cord. This is not the same type of cord that you use above water; it’s heavier and usually longer, which means it has increased electrical resistance, and requires specialized circuitry and controls to compensate for the underwater environment to ensure there is no flash misfiring.

TWO FLASH UNITS

Two flash units, properly located in relation to the subject and the camera, will generally provide even illumination without evident or excessive backscatter. The main flash unit, used to illuminate the subject, is the brighter of the two and therefore your exposure is calculated based on the amount of light it provides, while the second unit, generally intentionally less powerful, provides side or back lighting along the nature of fill flash.

The second flash unit can be activated with a remote light sensor triggered by the firing of the main flash. Light stands (and tripods) are not used underwater, so it can be helpful to have another diver hold and aim the secondary flash if you don't have an arm for it on the flash bracket.

Two flash units, properly-positioned, can make all the difference to a photograph.
Two flash units, properly-positioned, can make all the difference to a photograph.

PRE-TEST UNDERWATER FLASH

An effective way to exposure-test your underwater flash is to shoot color slide film in a swimming pool with the lights off at night.

  • Place a small brightly-colored object on the pool's bottom to use as your subject.
  • Maintain a constant subject distance, taking note of your exposure settings.
  • Then shoot, bracketing flash exposures by changing aperture settings.
  • If you carry a grease pencil and use the underside of an unneeded 4" X 6" or 8" X 10" print as your writing surface, you can record settings without having to leave the water.
  • Once your slides have been processed, check them against your notes to confirm the best exposure settings to use.

Pre-testing your flash will give you the confidence to properly illuminate complex scenes.
Pre-testing your flash will give you the confidence to properly illuminate complex scenes.

 
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