Canon 5D Mark III - Focus problems

Hey there Canon Shooters,

I am having a major problem with my 5D Mark III when shooting in low light conditions. This is in and out of the housing, more predominantly in the housing but I would put that stat down to the fact that I use it in water more often. My 5D Mark III is battling to focus i.e. low light but enhanced by artificial light (2 x fisheye &7000 Video Lights) - The the focal points all light up and the camera then does not fire. One can attempt to re-focus and the same will happen. In other circumstances the focal points appear in the view finder (black, not highlighted in red) and the camera will once again not fire. - Has anyone had a problem like this, and if yes, how have you been able to solve it?

I would ask how your focus is set. Are you using "single point", "tracking", etc.? I use single shot, only. You should also "move" the focus off of the camera shutter, onto the back focus. In this manner, you hit the back focus button and the focus "locks", if you are shooting macro/super-macro, you can then "rock" the housing, and when you're ready to shoot, then depress the shutter. I do not use the "tracking" focus at all. I'd say if this continues, you should send the body in for service, as I have found mine to do exceptionally well in low-light situations - in and out of the water.

It sounds like your focus points in your viewfinder are functioning normal but I'm not sure what's going on with your auto focus without seeing it. http://learn.usa.canon.com/resources/videos/tutorials/eos_5d_markiii_tutorials/mark_iii_intelligent_viewfinder-detail.shtml