Well, It's ok on Groundglass focusing screens.
Depends on the type of alcohol. Rubbing alcohol is
never okay to use, because (1) it is only 90% pure, at best, and only god and the company that makes it knows for sure what the other 10% is. For example, I read the lable on one bottle recently that contained balsams, oils, and other things sensible people don't want in their cameras -- the idea being to get the crud
out of the camera, not to put more in, and (2) isopropyl alcohol is a much more aggressive solvent than other types of alcohol and can dissolve and otherwise attack things you don't want it to -- and the
fumes can do this, not just the liquid itself. Ethyl alcohol, on the other hand, is relatively benign; I still wouldn't use it on a focusing screen though, even ground glass, when there are other, more harmless, solvents that work just as well, with far less risk to the camera. It won't hurt the ground glass itself, but it
will attack nearby balsam cements, for example. I'm convinced this is why you see so many old Voigtlander plate cameras with Skopars that have seperating lens elements.
Denatured alcohol and ethyl alcohol are the only alcohols I use in cameras and then only in
very small amounts under tightly controlled specific conditions -- none of which involve focusing screens. Truth be told though, if a focusing screen in an SLR gets something stuck to it, you're almost certainly going to be a whole lot better off learning to live with it than you are going to be if you attempt to clean it --
especially if it is a fresnel lens.
