"How much air do I have?"

"What's my depth?" 

"Oh, wait! What's that over there? It's a seahorse! How can I position myself to get the shot?"

Those are a few of the questions that go through my mind when doing underwater photography. Of course, once I find a subject, I have to do everything a topside photographer does, determine the right angle, exposure, shutter speed, etc. And underwater, light is especially critical. Believe it or not, too much sunlight is bad. Well, I guess that's true on land too, and why we don't take many photos at noon. But underwater, sunlight casts a blue-green color on everything that Photoshop can't completely undo.

The diver has to combat light from the sun by outshining it. That's why I have two powerful strobe lights on my camera housing and why -- if you were diving alongside me -- you'd see me continually adjusting the strobe arms to position the lights correctly. The goal is to get light on the subject without lighting up all the particulate matter in the water between the camera and the critter. Generally, that means not putting the light directly on the creature, but having it at an angle. And sometimes, as with this image, you don't need front or side light at all, but a backlight works.

That's where the assistant comes in. I've been lucky to dive with great dive guides. The one who led us in Bali was a fantastic photographer in his own right -- Alex Dharma. He not only found creatures for us to photograph, sometimes he also set up a scene for best photographic advantage. In this case, he angled one of his dive lights behind the seahorse and indicated that I should turn my strobes off. Then I lowered myself to the sea floor to get eye-to-eye with the seahorse and captured this image. There wasn't much sunlight in this instance because we were 90 feet down, so a single backlight on a small creature was enough to do the trick!

http://bit.ly/thornyseahorse