I went to my optomitrist to find out. that should give you an idea where to start, but it's best to try them first to find the right one. Some of my cameras have it built in and you can just dial in the best setting. If you have a camera that does this AND lets you know the level of adjustment, you can decide from there. If not, try them at your local camera store to narrow it down. If you carefully lift the rubber eye piece up at the bottom and slide it up, you will be able to remove your diopter to see what is installed. Probably -0.1 (standard). I bought mine used and had a hell of a time focusing. That was because I had a -0.5 installed. I removed it and can see just great now. The +0.2 doesn't work either. So I don't have any in it now, but even w/o my glasses I can focus better than before.