I've the 40 M-Rokkor, the 40 Nokton, and the 43 Pentax and like all three very well. The 43 is rather long physically and has a touch of barrel distortion, but is a lovely performer. The Rokkor is most compact and has served me very well for decades. The Nokton is fast and sharp, no complaints. I like 40's... as long as we're stretching to 43mm, should a bit more stretch include the 45 G-Planar? No, as it's actually 47mm, but it is incredibly good. How about the 40mm Ultron in various SLR mounts?