The K mount was the first "bayonet" type mount from Pentax and it has an automatic diaphragm that allows the camera to make a correct light metering wide-open and then closing momentarily the iris to the correct size during the exposure. It has the same function of the M42 lenses with "Auto" setting (those ones with a metallic pin near the mounting thread).
The KA mount was introduced along with the Pentax cameras with Program auto-exposure mode. It has a lock that fixes the aperture to a minimum and an electric contact to tell the camera it's set to Program mode. The only digital cameras capable of fully use KA mount lenses are the Pentax DSLRs.
When using a K-M mount on a Pentax DSLR, the camera will work on manual mode and aperture priority, but not program or shutter priority (For this you need a K-A type)
They can also be used on the Canon EOS range and all mirrorless system with the proper adapter of course, but, again, just in manual and Av Mode.
Back to the lenses...
Both 1.4 and 2.0 versions are although inexpensive, very good lenses, with excellent multi-layer coating and extremely easy to find on the 2nd hand market.
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| SMC Pentax-M 50mm F1.4 Do not copy without permission ! |
The F1.4 is very sharp from edge to edge from F3.5 and above and pretty decent even at F2.8, but has soft corners at F1.4 as expected.
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| SMC Pentax-M 50mm F2.0 Do not copy without permission ! |
The F2.0 version is sharp as the F1.4 at the same apertures, and it's lighter and less expensive.
You need to pay attention when buying one. Triple check the rear lens group, they are very prone to fogging due to the degradation of the optical cement used to glue one of the rear pair of elements. If they aren't absolutely clear, avoid it.
Otherwise, it's one of the best '50s bang-for-the-money you can find.
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