I found a cheap correction diopter (not right for me) and knocked out the glass, the rim is rubber-armored so screwed into my M3 eyepiece it protects my glasses and it's thinner (and nicer) than the stick-on ones (I had one of the DAG plastic ones, it would've needed pliobond to stay on). I can see the entire 50mm frame as long as I gaze straight at the rangefinder patch. Once I let my eye wander a little off-center one side or another of the frames completely disappears. The biggest problem is that for framing at longer distances I need to see another frameline-thickness outside the frames, and that is more of a problem. For me my M3 and its chrome Cron is an occasional-use outfit, which I bought and keep mostly for its historical significance. For general shooting I use M4 and M6, because I shoot a lot with the 35mm lens.