Flash Power and Guide Numbers

The power will have a large effect on the dynamics of your underwater flash photography. Power is usually given in the form of a Guide Number (GN) calculated in air (the power of the flash is reduced underwater). The exact amount will depend on visibility, particles in the water causing scatter, diffusion and blockage that affects the light reaching the subject and returning to the camera. For this reason an air guide number is your best guide when comparing flashguns with one another. They are used to calculate the aperture or f-stop required to enable the light to reach the subject at a given distance. The f-stop is calculated by dividing the guide number by the distance.

Let's take the example of photographing a Coral Grouper that won't let you get closer than 1.5 metres. The aperture required for the light to reach the Grouper on full power using a flashgun with an assumed underwater guide number of 14 will be f8 (closest f-stop to 9.3). For a flashgun with an assumed underwater guide number of 24 the aperture required will be f16. As a result a gun with a higher guide number gives you increased depth of field, greater working range and the use of low ISO. The maximum working range of the gun is affected by adding a diffuser.

Diffusers are used to spread the light with wide angle photography and to soften the light falling on the subject. When using a —1 diffuser, you lose one stop of light. For a flash with a guide number of 14 this means that the

FIG. 2.15 Epoque DML-2 Macro lens. Courtesy of Cameras Underwater.

aperture required for the light to reach the grouper on full power is increased from f8 to f5.6. This may require you to increase the ISO to get more depth of field, which may lead to more noise. Alternatively you need to get closer to the Grouper or use a flashgun with a higher guide number.

How the flash communicates with the camera also plays a role. Some operate on a slave function, whereby the light bouncing off the subject triggers the flash. Depending on how it is positioned and the coverage of the light sensor, firing may be intermittent, which can be very frustrating. A more reliable way of connecting the flash is via fibre optic cable. Fibre optic cables ensure firing every time and can be positioned anywhere without losing communication. Fibre optic cables have wet connections therefore reducing potential for leaks. The built-in flash is used to trigger the off camera flashgun.

To prevent backscatter a dark film is sometimes placed over the built-in flash allowing only short infra red light to trigger the off camera flash.

Some flashguns and housings will allow for hard-wire cabling. In this instance the built-in flash does not fire and there is no need to mask it off. Hard-wire cables are often a weak point in the system and are susceptible to damage at the connecting ends. Spares are highly recommended.

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