Quote Originally Posted by neuroanatomist
For 'alignment' to other common lenses, I would think. 15 and 85mm are obvious, as those are the ends of the range. 35mm and 50mm are 'classic' prime lens focal length, and Canon makes a very popular 24-70mm zoom lens.

This is what I always guessed- plus you want a geometric rather than arithmetic progression. (The sequence marked is fairly close to geometric, up to the last one.)