The Tamron Adaptall-2 55BB is probably the best catadioptric “mirror” lens for the average photographer. I’ve owned four CAT lenses, the 55BB, Tokina 500/8, Celestron 500/5.6 and Opteka 500/8. This Tamron is far and away the best, well worth having in ones kit. Sharpness is very good, I’d have given it an 8.5 if the rating were available. More @Wikipedia
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