Usually folks evaluate a lens on paper and then save for and acquire it, take it out and shoot with it. Along the way they debate the merits of X vs Y. I have been fortunate enough to acquire a FA 31 a couple of years ago. Wonderful lens, I like it very much - essentially what is there not to like. Late last year I had an opportunity to acquire a Contax Carl Zeiss 28mm f2.8 Distagon T* lens. I needed to change mounts in order to use it on my K20. After reading about Zeiss here, this was an affordable way to try one out, since the old Contax glass was about 1/3 less than a brand new shiny Zeiss. With it being 28mm it begs a natural comparison with the 31. They are both top of the line premium lenses from notable optical design houses.
Where I am going with this is that in comparing the two lenses (in the field [landscapes] - not a brick wall), I have seen that with the only difference being the lens - same ISO (100), same aperture (f4), nearly the same time (a minute difference between shot sequences) same landscape scene, I am finding that the Contax/Zeiss lens has shutter speeds about twice as fast as the Pentax FA Ltd.
I have taken all of the images on a tripod, 2 second mirror up, external shutter release, AP mode on each lens, multi-segment metering same K20 body, really no differences - other than swapping lenses. The other thing is that I have been shooting 5 image brackets at 0, -.5, +.5, -1, +1 ev, and the shutter times are really staggering between the two lenses, with about a minute between sets (just swapping the lens).
lens....0ev....-.5ev..+.5ev..-1ev..+1ev
CZ 28 1/15...1/20...1/10...1/30...1/8 sec
PK 31 1/8.....1/10...1/6.....1/15...1/4 sec
This observation is just not across one set of images, but many sets of images. Overall, there is a consistency in the numbers, however its not absolute, there are instances where the difference is both larger and smaller, and I have observed instances where the shutter times are essentially the same. On the average though, the difference is on the order of twice as fast.
My observation here is that it appears to me that the Contax/Zeiss glass in terms of the amount of glass used and the design applied, appears to have a built in efficiency to it, of about 1 f stop. That is, it is able to transmit more effective light to the sensor through the same aperture.
Now I know that most folks do not have access to similar lenses, but is there something that I am not taking in to account? My single optics class was over 40 years ago, and I didn't even keep the text book.