Originally posted by Laurentiu Cristofor My guess is that the ability to effectively mount/unmount M42 lenses was just not a design goal for the K-mount.
An' my guess is that's true.
It was 1972-73, folks! -- design goals were being developed for the next generation of Pentax lenses which were introduced in 1975. The KA2 lens mount wasn't even a glimmer in the designers' eye; the capabilities of the FA lenses and body types were as yet undefined and largely unforeseen. (Which desktop computer and wireless phone did
you have then?)
The immediate design goals for the K-mount were to:
- increase the mount diameter to allow electric AE contacts, "faster" lenses and eventually some sort of connection for a mechanical auto focus mechanism.
- convert from the
linear diameter aperture adjustment mechanism to an
area-rule aperture scheme to accommodate the needs of simple, open aperture, matrix metering.
- design a mount with
more precise indexing than the M42 thread mount was capable of to satisfy the requirements of items 1 and 2. (That the bayonet was quicker to mount was a nice touch too.)
The diameter of the existing M42 mount simply did not provide the dimensions needed. Other manufacturers faced with the same challenge mostly chose to completely abandon compatibility with legacy lenses in the interest of simplicity and cost.
The fact that Pentax ended up with a mount with the same registration distance that allowed the older lenses to be
mechanically adapted
within its diameter is a plus.
Providing an M42 > PK adapter was a marketing tool that allowed pro/business users worldwide (and a much smaller pool then of "wealthy amateurs" who could afford a LOT of lenses) to spread the cost of transitioning to the new lenses and bodies over a much longer period of time.
Employees in 1975 didn't envision that a consumer market population would be calling Pentax Corp. devious, bumbling, incompetent conspirators on an internet forum today because they couldn't use cheap, 40-year old lenses on digital SLR bodies and had no way to define such a feature.
Consider this: even if there was a magic adapter to somehow convert an old Super Takumar 50mm/1.4 into a virtual DA auto-focus lens there'd still be the mechanical issue of linear vs. area rule aperture operation and its effect on exposure to contend with. Or were they supposed to have anticipated a software program to translate that effect too?
Pentax DID make it easy to use legacy M42 glass though. It's called M mode. It's just not as convenient as with a ol' Spotmatic 'cause that silly, damn internal flash bump hides the aperture ring. That's
my only gripe.
H2