Know your stuff
Using artificial light (and how to balance it with natural light)
The best underwater shots appear naturally lit because of the balance of ambient (available) light with strobe or flash. Getting this balance right is key.
In the middle image above, the colours are washed out and the background too dark because the ambient light reading has not been accounted for.
The camera’s aperture controls the amount of light hitting the sensor at any given moment, while shutter speed controls the length of time the light hits the sensor. However, flash is a very short burst of powerful light (1/2 000th to 1/15 000th of a second). Flash sync speed is the fastest shutter speed you can use with a flash. This speed differs from camera to camera but is usually in the 1/160 to 1/250 sec range. This is the shortest period of time the entire sensor will be visible. At faster shutter speeds, parts of the sensor will be covered by the shutter curtain as it moves across the sensor.
Because it’s so quick, shutter speed will have no influence on the amount of flash light hitting the sensor. So the only way to control flash brightness is by using your camera’s aperture and ISO settings.
The key to balancing flash and ambient exposure is to first read your ambient exposure and then change the aperture settings to correct for flash exposure. Finally, bring back your ambient exposure by changing the shutter speed.
Also remember that distance from the subject plays a big role in the strength of your flash. As a rule of thumb: double the distance away, half the flash power
– and the same in reverse (half the distance, double the power). Try changing your position, moving back or forwards, to get the best-lit shot.