" The M42 lens mount is a screw thread mounting standard for attaching lenses to 35 mm cameras, primarily single-lens reflex models. It is more accurately known as the M42 × 1 mm standard, which means that it is a metric screw thread of 42 mm diameter and 1 mm thread pitch. (The M42 lens mount should not be confused with the T-mount, which shares the 42mm throat diameter, but differs by having a 0.75mm thread pitch.) It was first used in Zeiss' Contax S of 1949; this East German branch of Zeiss also sold cameras under the Pentacon name; after merger with other East German photographic manufacturers, the name Praktica was used. M42 thread mount cameras first became well known under the Praktica brand, and thus the M42 mount is known as the Praktica thread mount.[1] Since there were no proprietary elements to the M42 mount, many other manufacturers used it; this has led to it being called the Universal thread mount or Universal screw mount by many.
The M42 mount was popularized in the United States by Pentax; thus, it is also known as the Pentax thread mount, despite the fact that Pentax did not originate it. . . .
https://en.wikipedia.org/wiki/M42_lens_mount
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The Evolution of the Pentax K-mount
Technical Information and a Little Bit of History
By PF Staff in Articles and Tips on Dec 29, 2014
Read more at:
https://www.pentaxforums.com/articles/photo-articles/evolution-of-pentax-k-m...#ixzz55A4EZYDH
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Pentax Lens Compatibility Chart
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Pentax bayonet lenses labeled "SMC Pentax" are referred to as "K" lenses in the table below. Other Pentax bayonet lenses have the lens designation as a part of their name as in "SMC Pentax-FA."
Note that "star" lenses work like their "non-star" counterparts: DA* lenses work like DA lenses, FA* like FA, F* like F, A* like A, and M* like M.
Exceptions:
FA and F "soft" lenses behave like M lenses in terms of metering.
The Pentax Camera Lens Compatibility Chart
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Pentax K-Mount Lens Series Explained: The differences between various Pentax lens series
Pentax K-Mount Lenses Explained: The differences between various Pentax lens series