Originally posted by farfisa D FA is the designation for FF lenses that have been updated for digital, so the 'cropping factor' is not a consideration here.
As for how it was updated, the D FA features an "optimized lens coating and positioning of optical elements" for the special characteristic of today's image sensors. (quoted from Photozone.de) The newer coatings may give increased contrast, but I'm not sure what changed re the "positioning of the elements."
Also, you'll notice that the FA doesn't need a hood, because the front element is recessed, while the D FA does come with a hood--this probably explains the difference in length.
I really think Photozone is f.o.s. about the coatings. The only truly new digital era coating from Pentax is Aero Bright and the SP (Super Protect). Given how photozone doesn't seem to like Pentax very much, I doubt they would tell them much about the coatings. I say that because there is very little information available on any of their coatings. from 1952 to present and can't imagine Pentax or Hoya being forthwith. The Ghostless Coating used in many of the FA*, DA* and all the LTD lenses as well as the FA 35/2, DA 35/2.4 has been around since 1992 and used first in the FA 43 LTD.
That said, the biggest change in the D FA WR was the aperture blade mechanism and removal of the aperture ring and the seals.
All for af versions have FREE. However the major differences other than the barrel lie with the arrangement of the elements. The F and FA lacks the Ghostless Coatings, but I suspect the D FA and D FA WR have it. It is possible that the last years of the FA had it added.
The F and FA are:
Both D FA are:
Images are compliments of Dimitrov at:
Pentax Short Telephoto Prime Lenses