This technique may be widely known, but I've not seen it posted here before, and discovering it has saved me a lot of aggravation. Canon DSLRs, at least the 20D, 30D, Rebel and 5D have Custom Function 4 available which enables you to decouple the focus lock from the shutter and auto-exposure button. By setting CF 4 to either 1 or 3, you can use the AE Lock button * as the auto-focus control. Ikelite housings, at least, have a controller for this. By using this to focus, when you let up on the button, focus is locked, which means you can now use manual focus if you are not able to achieve auto focus for whatever reason - dim light, poor contrast, etc. Any of the auto focus EOS lenses should be able to be switched to manual focus with this technique, assuming the port has a focus control.
Ed Druy
