Originally posted by axl how does 77ltd compare to those 85s?
There have been many posts about this over the years, which I have followed intently. The best of these had side-by-side shots, which have unfortunately now evaporated into the digital ether. My information is second-hand, but comes from the best sources.
The FA*85 is specifically designed for portraits and so is optimised for that distance and a very particular focus. Basically, it front focuses so the subject is nearer to the back of the depth of field. This means that lovely bokeh begins directly behind the subject. The necessary by-product is that sharpness is sacrificed. This is known to all who have compared with the A*85, a sharper lens at the large apertures it would likely be used at (up until at least f/4, some say f/8).
The FA*85 has an 8 element in 7 group optical formula while the A*85 and FA77 have an almost identical 7 element in 6 group construction. They are also two of the few Fixed Rear Element Extension (FREE) focusing lenses Pentax has made. But the FA77 is half the size, half the weight, more convenient and looks better too (IMO). Some even prefer the bokeh. Certainly in this regard there is little to choose between them. All of this has been achieved by sacrificing two-thirds of a stop, a most admirable compromise, since few would use f/1.4 in practical use.
FA77 also has better control of aberrations. And is a better field of view on crop sensor, IMO, being similar to 120mm FF. (Note that the lens is closer to an actual 80mm than what it is rated at.)