A black background is a nice, artistic way to isolate a subject. How do people get those black backgrounds in their underwater photos?
The important thing is to try not to have anything in the background. This is often accomplished by "shooting up", getting below the subject, which may be located on top of a rock, on a piece of coral, etc. However, this is not always possible, but there are ways of dealing with this.
Use as fast a shutter speed as possible, which is your strobe sync speed, around 1/200th - 1/250th on a dLSR, much faster on a compact, I suggest using 1/500th.
Use a small aperture. A combination of a fast shutter speed and a small aperture will block out any ambient light
Using a fast shutter speed and small aperture are more important if you are dealing with brighter ambient light. However, don't forget the most important thing, minimizing what is behind your subject, an maximizing the ratio of (subject to background distance) / (subject to strobe distance).
If there is a background behind your subject, (which is often the case, strobe positioning can often be used to darken the background. Having your strobes at the sides, pointed slightly inward towards the camera housing will work. The 10-2 position can also work. See the article on underwater strobe position diagrams.
Don't underestimate the importance of strobe position when trying to get a black background, if there is anything at all behind your subject. If there is nothing behind your subject, you can use a strobe position that best minimized backscatter (strobes out a little wide, pointed slightly out).
And please don't toss an octopus in the water column for a black background!
This Janolus nudibranch has a nice black background. I made sure nothing was behind it, got close, and stopped by aperture down.
The subject here is emphasized because of the blackground behind it. This is a small cowrie on a soft coral, photo by jeff de Guzman, uncropped.
A black background helps make this Cuthona nudibranch really stand out. F22, 1/200th
Wire coral goby with black background, Bali. F20, 1/200th, ISO 320. I got low and "shot up" to make sure nothing but water was in the background.
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