Find focus and effective range of single focus camera

Question

I have a cheap camera that is supposed to take videos underwater. It is a camera in a case that is set to a single focus. I checked the specs and it is a manual set focus camera. When I asked the supplier, they told me that I can't change the setting anymore since it's already in its waterproof case.

I'd like to know if there is a certain quantitative test that would help me find the range underwater where its focus is set to capture images for. So far, all of the tests I've found online are for DSLR cameras and I don't know if they would be applicable for my camera. I know that I could just take pictures until one shows up clear but I would still like to do a proper test on it.

Thanks in advance.

Answer 1

The apparent distance of an object in a water viewed from air ,d_prime, equals true distance* 1/1.3333 (index of refraction of air/ index of water).

So an object d feet away will appear to be 25% closer in water. So you can just find the focus distance on land and then make the adjustment.

Answer 2

I suppose there's a simple way. You could bring a measuring tape or a long ruler down with you underwater. Hold the ruler out in front of the camera and observe at what distance does the markings of the ruler become clear. That should give you a fairly accurate measurement of the focusing distance in water.