My experience, for what it is worth...... Years ago, (pre-digital )I had a Minolta SLR camera with several lens including a Mirror lens - a 500mm one in this case. It was not auto focus. I found it really difficult for some reason to get an exact sharp focus all of the time. More often than I would like I was just slightly out. A zoom lens seemed to be much more forgiving in this regard- easier to get an acceptably sharp image. Interested to know if anyone else had the same experience. Presumably there are autofocus ones nowadays that get around this issue.