Lens element and group configurations can be identical, but designs may still be significantly different due to different glass formulas surface figures.
Bob...
Indeed. This is the case of K17/4.
Pentax public presentations of lenses (as reported in Japanese english-written Camerart magazine, for example) may be the best reference on that subject. When Pentax presented the new K mount equipment (Camerart july 75), they implied K lenses were the same as previous SMC-Takumar except for the four mentionned previously. Contrary to the K17/4 case, changes for 28mm 35mm and 300mm lenses were obvious: more lens elements (28mm, 300mm), different surface curves (35/2).
K-series trivia:
SMCT 20/4.5 never made it to K and was replaced by K20/4. K50/1.2 was the only other completely new design at the time of the K-series introduction. K28/2 and 28mm-shift were also announced but it took almost a year before they were released (mid-76).
At the end of 1976, the M-series equipment was presented and some K lenses were dropped soon afterwards: the 24/3.5, 28/3.5, both 35mm, 105mm, 135/3.5, 200/4, 50/4 & 100/4 lenses were then only available for about a year. The other original K lenses (50/1.4, 55/1.8, 85/1.8, 120/2.8, 150/4, 85-210/4.5) were dropped not much time afterwards. All K lenses are hard to find. Normal lenses are easier to get by but usually come on a K-series body.
K55mm f2 is a special case, it was only available with the first K1000 bodies, for maybe a few months at the end of 76. It is the same lens as the 55/1.8, it just does not open to 1.8. You may be able to take out a ring inside the lens to get the 1.8 aperture, but I would not do that as the lens is a "collectible".
K50/4 is a very sturdy lens with a deeply recessed front element while the M version is much lighter with less internal baffling. For studio work, the K version is worth having.
100/4 was the only lens that was made it unchanged optically from pre-SMC Takumar (bellows) to A-series. Mechanically, the M and A lenses are nearly as good as the K version.
It has been said by Pentax that for the K-series lenses, SMC was added between the elements of a group. This was a very very thin improvement... But good enough for some adds.
Andre
