Hi - it is optically identical to the RZ 150 3.5 - the same 6 elements in 4 groups. It has a good reputation for sharpness. I've always felt there's rather an unnecessary cluster of lenses between 90 and 180 for the RZ/RB series - at one time a 90, 110, 127, 150 and 180 (and that's ignoring the 150 & 180 SF and 140 Macro). If you don't need speed, the 100-200 zoom seems a rather tempting alternative as I've only ever heard positive things said about lack of distortion, contrast and sharpness. Probably weighs a ton though....