Setting fixed focus on my UW camera

[jules59]

I have a Sony P200 compact digital which I use in a Ikelite UW housing.
In video capture mode, to stop the auto focus from zooming in and out onto things moving into the field of view, it is possible to fix the focus at 0.5m, 1m, 3m, 7m and infinity.
Should I set the focus distance at the actual object distance (assuming I can guess it accurately) or at my perceived object distance (ie closer than the object really is)?
Does this difference really matter?

Jules

[Ben Panter]

Hi Jules,

Short answer -- apparent, through-the-lens focal distance.

Long answer -- The camera will predict based on air rather than air/glass of port/water refraction. The best way to calibrate for your particular system will be to get a meter stick and take it underwater with a few targets at known distances (suggest a swimming pool for this) then experiment. Take down a slate and mark out what physical distance corresponds to what focal distance setting in your camera.

Should be able to do what you're after then...

Best of luck,

Ben

[jules59]

Thanks
I'll take my camera and tape measure to the pool next week and put it to the test. No doubt I will get alot of comments from the rest guys there.
Jules