shooting a black background with Macro!?

[TheSmoothDome]

Not quite. Shoot at the fastest shutter speed you can sync with and the smallest aperture that you can to still get your strobe to light the scene. In general, the faster the shutter speed the less ambient light gets in. The smaller the aperture the less ambient light gets in. Shoot upward to clear water, and set your strobe to whatever power you need with the fastest shutter speed you can and use an fstop that you want to inversely control ambient light and depth of field. Meaning, small fStop (higher numbers) means larger DOF and less ambient light.
Bill

My Canon 50D and my previously owned Canon 450 couldn't sync with my strobes past 1/250th. A lot of my macro that produces good black backgrounds are shot at 1/125th with an apeture of 22. Any faster shutter speed or smaller apeture and I tend to be underexposed even in good conditions. Having bright light above, I might get an extra stop though.

 

[ClosefocusWA]

Real key is to shoot from an angle where the background is open water.

Any sand, reef, etc. and the strobe will light that and ruin your black background.

 

[blibecap]

I shot this last night in a indoor pool. ISO-200 F-22 1/200 tokinia 10-17 and 1.4 tele nikon d90 Ikelite housing 5503.55 port

It was fun and interesting to see how black the water can get.

Here is another picture shot at ISO-200 f-10 1/200 tokinia 10-17 and 1.4 tele nikon d90 Ikelite housing 5503.55 port