Thread: Asahi Pentax S2
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Old 01-23-2020   #16
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Here's a link to a very comprehensive site describing early Asahi Pentax cameras and lenses.

Also, it should be noted that M39 and LTM are not the same thing. LTM is a 39mm thread mount with an inch-standard 26tpi thread pitch (borrowed from microscope standards) which was used by Leica's thread mount RFs and the numerous clones. It is often incorrectly labeled as M39. LTM has a register (flange to film distance) of 28.8mm. There was an obscure SLR mount that used the LTM diameter and pitch specs but used a 45.5mm register.

M39 should only be used when referring to either of the pure metric mounts which have a 39mm diameter and a 1mm pitch. This has been used by a few RF cameras with a 28.8mm register distance and on a few SLRs when it has a 45.5mm register. This narrow mount severely restricted lens design and was replaced by M42 with a 42mm diameter and the same 45.5mm register.

The SLR M39 lenses can easily be adapted to M42 with a simple adapter and will retain infinity focus. The RF lenses, whether M39 or LTM, can be adapted but are usable only for closeup and macro work.
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