The FA20 is a nice small lens.
Here it is compared to a Canon FD20 (roughly same lineage type lens) and the size difference is apparent.
and we are not even talking A20 or M20 here.
1 filter size smaller a lens.
Goes to show off the design considerations Pentax always gave to its lenses in terms of optical performance and lower encumbrance.
I've also compared these 2 lenses (on FF A7).
Pentax has more field curature.
Thats good and bad.
Good in the sense that images esp with the wider apertures have more gradual fall off between in/out of focus.
More dimensional.
Bad in that as flat field lens, care must be taken to get things 'all sharp'.
The FA20 also has more contrast and a 'pop' to it because of it.
Better richer colors too.
The thing about it is field curvature.
On FF (at least on a A7 with think sensor stack well documented to be problematic ), the edges will be much worse than the center.
I basically needed a f11 (or f9, f10) and focus using the off center to make sure everything was a sharp as possible.
Here is the trial shot at f11 and focused off centre.
Larger size sample on the flickr.
Its a nice lens for sure.
I don't know about the FA20-35.
But I am certainly thinking of the FA,A,M20 to add to the collection.