Originally posted by Blue We were talking about Pentax lenses. Pentax nor Asahi Optical ever put a 560 lens out before. Plus this Pentax lenses was announced to be 5.6 where as the Telyt-R was available in f4 and f6.8 models. It still makes it a more odd focal length relative to 300, 400, 500 and 600mm lenses given all the major makers have produced those FL. Hirakawa Jun explained the reason behind the 43 and 77 but he didn't design the 31, but yes they are unique to Pentax. The 43 is the diagonal of the full frame 135 film and the 77 is a lucky number between 70-80.
Pentax nor Asahi never put a 60-250 lens before, or a 35mm macro, or a 12-24mm, or a 14mm, a 50-135, and many more lenses. Some of those designation are simply old lenght reduce by the crop factor to allow the same file of view, some are due to technical reasons to allow better IQ or to reduce costs... in the end there are not worng of right focal lenght... in a range of xooms you have everyone of them, and use them, at least I don't jump from 85 to 135 to 200 on my 60-250, avoiding the in-between lenght.
By the way, 560/5,6 menas the frontal lens is 10cm in diameter, and probably the choice of the focal lenght was to have not too big and not to expensive glass pieces to produce, to keep down the final cost.