[Dewman]

I recently came into possession of a very nice old M42 mount Pentax-M 135mm lens. I sent off for and received an adapter, but when installing the combo, I discovered that the lens will not focus to infinity. If won't even come close! More like about 6-8 feet is maximum. It's as if an extension tube had been installed. After looking at the adapter and studying it for a while, it would seem to my untrained eyes that the lens, when attached to the adapter, SHOULD be set back INTO the body somewhat, like it would if the body had a threaded mount. Does that make sense? In other words, the lens, when fitted to the adapter is far to far forward for it to focus properly. Would I be correct in assuming that I got either the wrong adapter or one that was not the one I really needed? Or was it just a cheapo and I got my money's worth?

[paulh]

If it's a flanged adapter - then no, it will not let you focus at infinity. The correct ones will look similar to the genuine Pentax m42-K adapter (sorry, don't have an image of one handy, but easy to look up).

[Rimfiredude]

The Pentax Brand works correctly. Any of the others have their problems.

[jatrax]

The back of the lens should (must) fit flush to the mount of the camera. If it does not then it cannot focus to infinity.

There are adapters (called flanged adapters) that prevent this. They are still useful for bellows and macro applications but not for general use. So if your adapter, when mounted on the camera sits proud of the camera mount then it will not work properly.

M42 was used by Pentax prior to the k-mount and they retained the precise same flange distance from M42 when the k-mount was designed.

The Pentax made adapter is well made and always works, though expensive. The third ones usually work but are often cheaply made and case cause issues. And of course the ones with flanges don't work for your application at all.

[manntax]

M42 mount Pentax-M 135mm lens

this is self-contradictory. Pentax -M lenses are with bayonet, K mount. M42 is screw mount, for older Pentax cameras. - Can you provide a photo of both the lens and your adapter ?

[Dewman]

this is self-contradictory. Pentax -M lenses are with bayonet, K mount. M42 is screw mount, for older Pentax cameras. - Can you provide a photo of both the lens and your adapter ?

My bad.... of course it is NOT a "M" lens, it's a M42 mount.... but the reason I'm experiencing what I am is now very apparent. Too bad, 'cause it's a dandy little lens. The test shots, although quite close, look very sharp. A well made lens, to be sure.... but, not really my cup of tea. Anyone interested? If so, PM me and we'll talk.