[Laurentiu Cristofor]

Perhaps, but he usually leaves nice comments on my Flickr stream

It occurred to me a few minutes ago what he may have been referring to. Most other camera makers chose to locate their electrical contacts inside the mount rather than on the mount face when they moved to support more sophisticated features. The same narrow diameter lens adapted to Canon (for example) will not have the issues of exposed contacts and because Canon has a shorter registration distance, the adapter can include a generous wide and very stable flange as well.

Yes, I commented a couple of times on this and I think I'll write a longer blog post as well. I think the K mount keeping the same flange focal distance as the M42 mount is a weird engineering decision - I don't know if they wanted to handicap the use of M42 lenses but that was the end result. All these little annoyances around the use of M42 lenses on K mount can be tracked back to this.

[stevebrot]

This shows one of the two adapter clip holes

So it does!!! That is one small diameter lens.


Steve

[stevebrot]

I think the K mount keeping the same flange focal distance as the M42 mount is a weird engineering decision

Maybe not as weird as you might think. Back-in-the-day, mirror length and clearance for fat lenses was a big issue and those cameras with short registration distance often had compatibility issues with certain T-mount lenses. You needed a longer mirror to provide coverage for the longer teles of the day, but to support that with a short registration distance was sometimes almost impossible. (Think unusual mirror travel strategies!)

Translation...there is a reason why both Pentax and Nikon stayed on the long side of things.


Steve

[Laurentiu Cristofor]

Maybe not as weird as you might think. Back-in-the-day, mirror length and clearance for fat lenses was a big issue and those cameras with short registration distance often had compatibility issues with certain T-mount lenses. You needed a longer mirror to provide coverage for the longer teles of the day, but to support that with a short registration distance was sometimes almost impossible. (Think unusual mirror travel strategies!)

Translation...there is a reason why both Pentax and Nikon stayed on the long side of things.

Maybe figuring out how to solve such engineering problems is what allowed Canon to leapfrog them both. Their FD mount, introduced in 1971, 4 years before the K mount, had even shorter registration distance than all extant SLR mounts. It could be done back in that day.

BTW, I just finished writing my blog post on this topic.

[Lloydy]

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/113150-trap-fo...mar-135mm.html

I posted this a while back, it might be useful ?

[lytrytyr]

I think the K mount keeping the same flange focal distance as the M42 mount is a weird engineering decision - I don't know if they wanted to handicap the use of M42 lenses but that was the end result.

The flange distance worked well for M42,
so keeping the same distance for the K-mount made sense,
as well as allowing a direct conversion of lens designs from one mount to the other,
like Takumar 55/1.8 to K55/1.8.

The shorter Canon distance seems to have led
to mirror fouling problems using off-brand lenses on that system.