Originally posted by RioRico I still think the question is open as to why the specific sweet spots? 35's and 50's and 55's and the not-so-rare 58's; then 85's, 90's, 100's, 105's; then 135's; then a few (expensive) 180's; then 200's. Why no 44's, no 75's, no 120's, no 155's? Tradition? Demand? Optics? It's not like these are impossible to make. P&S rangefinders often had 45's. MF folders had 65's, 75's, 115's. Why? WHY?!?!? Tainted minds want to know...
OK lets look at some pentax past and current offerings.
they made takumar (M42) and SMC (K mount) 120mm, and 150mm lenses
they still offer the 77mmF1.8 (close enough to 75mm for you?) as well as the DA70
I think if you look at the full range of focal lengths, you might be surprised that they seem to offer lenses in steps that typically add 30-50% to the focal length,
What you are suggesting is that every maker make lenses in 1mm focal length increments so you have infinite selection. well they do, it's called a zoom. for thosw who prefer to zoom with their feet, and have an acceptable working distance or FOV or Magnification selection offered by the differing focal lenghts offered, in a lens line up you dont need to have one lens at every mm increment.