Has anybody ever tried to compare the two lenses?
In general I appreciate very much the early Pentax-K series, that in general was built without cutting corners, both mechanically and optically.
IMHO few focals have increased their performance from K to M and A.
There are exceptions, like the A Star's, the Pentax-A Macro 2.8/100, and the Pentax-A 4/200mm, but most other focals/types have worse mechanics and simpler optical layouts.
In this case the two designs are not very different: the earlier model has a cemented doublet at the back, the newer version (M and A) has two air spaced elements.
In theory it looks like the advance in coating technology allowed for two more glass-to-air surfaces and and a higher degree of freedom in aberration correction (theoretically an air spaced doublet is more efficient than a cemented one).
In practice... I have no idea
The old Auto Takumar 1.8/85mm, with five elements and released in 1960 (!), is almost as sharp as the Pentax-M 2/85mm, has better bokeh and overall rendering, and shows almost no fringing. In a few words, it trashes its younger sibling.
When it comes to 100/105mm lenses, I really have no idea. I guess the only way to know is to ask those who have/had the chance to actually try both lenses.
I have a similar problem finding a cheap AF zoom that goes down to a minimum focal of 24mm.
I have the FA 24-90mm (but I don't have it with me), and I recently eyed a nice F 24-50mm sold for a nice price.
Is really the latter better than the former?
The zoom range is not so important, I just want a wide angle zoom with good sharpness and controlled CA.
The user reviews are not easy to interpreter, and there is no direct comparison...