over/under (underwater) focus

I've recently purchased an underwater housing for my 5D, and with it, got the 8" dome port which allows

me to shoot over/under photos. It's a cool effect, but I'm stumped how anyone gets sharp focus both

above and below the water, as the water has its own optical properties. So, here's an example shot. What

would I need to do such that the person is in focus both above and below the water? Shoot at a (much)

smaller aperture?

Canon 5D, 17-40 F4L at 21mm, 1/125 at f/4, ISO 1600.

Well, since no one else has answered, I'll give it a shot. But I've never taken any underwater pictures.

The reason lenses "work" the way they do in the first place is that light bends at the interface between two media, like glass and air (I realize many lenses aren't made out of glass, but it's the best I can do for now). The amount of the bend will depend on the ratio of speeds of light in the two different media. For air/glass, this ratio is about 1.5 or more, for water/glass it is typically less than 1.2.

So the focal lengths in the two media will always be different. Your suggestion of using a smaller aperture will certainly help, since there is less bending generally near the center of the lens, but it will always be a palliative, and not a cure.

I think. Good luck.

You could try using a split field close up lens:

http://www.cokin.com/filtres6.html?=#111

You may want to do some research as to which strength and focal length combination would be most effective.