Originally posted by Docrwm There's a 21 and a 35 so a 28 would be 7 from each - right in the middle.
A couple mm matter more at shorter than longer FLs. Often the next jump beyond 35mm is to 45mm (I have a pancake Chinon 45/2.8 in PK-M). The logical place for a lens between 21-35mm is at 26mm. I don't know if that's an easy-design FL. I know Tokina had an M42 25/3.5 that wasn't large nor bad optically (although both my copies had stuck apertures). And of course Meyer-Pentacon had a 29mm.
I'm also curious as to how accurate are these specified focal lengths and apertures. I've read that the FA50/1.4 is actually more like 52/1.5. Can we really distinguish FOV differences between 19-20-21, 24-25, 28-29-30-31, 35-37, 43-45, 50-52 (I've had Industars and Ektars marked 52mm), 55-58, 80-85-90, 100-105-110, etc? Wouldn't it be funny if the FA31Ltd is actually a 29? What's the tolerance in specs?
NOTE: Various lensmakers churned out both consumer and professional copy lenses marked as 49-50-51-52-53-56mm -- I've had lenses with all those focal lengths. I've also seen the same lens in different brands marked as f/3.8, f/3.9, f/4. I suspect that's a marketing ploy, eh? And if we're marketing... how about if Pentax is known as the brand with the weird focal lengths? 14-21-26-31 certainly make a jarring pattern.