You don't make it clear what you are trying to focus. Is it a manual focus camera, or a grain focusing magnifyer used in the dark room when using an enlarger.

If it is the latter, most grain focussing devices can be adjusted for defective eyesight so that may cure the problem. If it is a manual focus camera screen you are looking at, it may be possible to buy a correction lens to bring the screen into sharp focus. Some cameras actually had a focus adjuster for sight defects built in.

It may help if you give us a better idea of what you are using to make sure we are talking about the same thing.