I was browsing around, looking at the various lenses available for my AE 1-P, and saw 2 different types of 500mm lens. One was a telephoto, the other a telephoto mirror that was about half the length, give or take. What's the difference in the two? Are they basically the same, other than the aperture range? I didn't see anything that explained the design difference a mirror reflex lens would have over a long lens. Somewhat curious now.