These guys are very cheap. Why not to use a manually-controlled camera instead of looking for a fancy solution.

Otherwise, you may need to dig deeply into the particulars of the equipment driver.

Well, this particular camera is not fancy at all. Moreover, most of the researchers have to deal with the equipment that the lab provided them. Unfortunately, a manually-controlled camera is not available.

Thanks Bruno,

It is clear. Unfortunately, I cannot advice on the camera. My experience, however, shows that it is easier to spend 10$ (even out of my own pocket) that spend 1 month figuring out a problem of the kind - a very particular problem with minor overall consequences.

An alternative (brute-force - be careful!) solution is either physically prevent focus change (stick tape around the lens) or remove the case and remove a wire froma focusing mechanism. It may be faster than dealing with programming matters.

A lesse-brute-force solution is to help the camera to focus nicely - add a lot of light, draw a red cross in the centre to aid focusing.