Hope that subtitle doesn't attract the wrong sort of people!
Having a flash on your camera gives you a number of options beyond shooting available light which I discussed earlier in the blogs about Custom White Balance and shooting RAW.
The first option is to use the flash on the camera itself to illuminate your subject. Most housings for compact cameras and some housings for mirrorless cameras have windows to allow light from the flash out, they usually also come with a diffuser to soften and spread the flash light. With any flash you need to be close to the subject because the water will reduce the distance the light will travel making it much less effective than it would be in air. But you don't necessarily have to be as close as this rather scary shot of me taken by Anne using the built in flash on a Canon S95 camera.
You may also have to deal with shadows caused by the flash being close to the housing port. But even a small built in flash will give out more light in an instant burst than all but the most powerful video lights. It's the inability to change the position that the light comes from that puts the built in flash on a camera at a disadvantage. This said for conditions where available light isn't good enough to get the shot it's worth having a try with the built in flash if that is all you have to work with. A useful tip is that you'll get more even lighting of your subject shooting portrait with the flash at the top than when shooting landscape. Here's an image of a Spotted Moray taken using built-in flash by me in Bonaire, again using the Canon S95 in a Canon underwater housing.
The second option puts a camera with a flash even further ahead of the action cameras. That's in allowing you to trigger a strobe.
Before we go further let's clarify this. A strobe is the name given by underwater photographers to a separate flash designed for underwater photography. Like the Inon S2000 pictured below.
Having a strobe when combined with an good arm system to mount it on gives you a positionable light source that has more coverage and power than a built-in flash. This instant burst of light allows you to illuminate the foreground of your images while using your shutter speed to control the lighting of the background in your image.
The shot below was taken in this way using a pair of strobes.
If you use video lights you have to use your camera settings to control the exposure of the whole image which means that ISO's are often raised, shutter speeds slower and apertures wider. This contributes to why images taken with video lights rarely appear as sharp and crisp as those taken with strobes.
A pair of strobes isn't necessary for all types of underwater photography. You can get good results using a single strobe especially in macro photography where a single strobe gives a lot of scope for different techniques. The image below was taken using a single strobe fitted with a snoot to create a small area that is lit.
Strobes can be triggered in a number of ways. The most popular now is using a fibre optic cable to carry the light from a cameras flash to the sensor on the strobe. With cameras that have a hotshoe either instead of or as well as a built in flash, electronic cable connection can be used. This does limit which strobes can be used and means that a connection has to run through a port on the housing but means that you don't have to use battery power firing a flash to trigger a strobe. It is now possible to buy strobe triggers that connect to the camera hotshoe and produce a flash of light by LED to allow fibre optic cables to be used without a built in flash.
Using strobes is a big subject and I'll cover more in a separate set of blogs in the future. Alphamarine Photography is a UK dealer for Inon, Nauticam, Fantasea and Big Blue Dive Lights. If you'd like advice or are interesting in buying equipment or booking on one of our workshops get in touch via our Contact page or join our Q and A group on Facebook