Problems w DSLR not autofocusing under low vis

[Larry C]

To get a better edge for focus and avoid backscatter, try putting your focus light directly above the subject and pointing straight down.

 

[kaaralex]

Thanks everybody for the hints! I quite like the idea of a focus light, preferably attached to the camera and not occupying my hand and especially a one that can be used both for autofocus, as well as for - very (VERY) ocassional - video shot. ("VERY" coz I do not have any intention to start making UW videos, one reason being that focusing on my DSLR (7D) is sooooo cumbersome.)

I will leave the process of selecting a right one to longer winter evenings.

One last question or thought on the AF light creating backscatter though: Larry C, what do u mean by "try putting your focus light directly above the subject and pointing straight down." ?

Unless i work out some modification of my housing, the only possible placing at this moment is to one of the arms of strobes - there is a small hole with thread right towards end of the arm, so i can attach it there. I would guess that u want to position the light further from camera and point it not directly towards to subject in order to avoid the backscatter?
Logic would say that the light of strobes is much stronger, so when exposing the pic at the right aperture and time for the strobes, the weaker light from the AF light should not be captured .. however, i checked two or three lights suggested in the thread, and they seem to be quite bright - 550-600 lumens is just half of the output of those 1000lumen HIDs which my buddies use, and once their beams find way into the pic, it is ALWAYS more than annoyingly visible there...

 

[Doubler]

Kaaralex,
Which housing do you have? I have an Ikelite and it has a 1/4 20 inch thread top dead center. I screw in a ball and attach my Focus light there. No modifications required. The Big Blue Focus light goes off automatically with strobe fire so no annoyingly visible light.

 

[craracer]

Strobe alignment will determine the presence/absence of backscatter. Strobes are much brighter than a focus light.