this may be too late for the OP
but I like Pentax because of the ability to use the " latest and brightest " for almost all of the Pentax bodies as well as vintage lenses
look at these charts for details:
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 unless they are singled out in the table below. So for compatibility of a DA* lens, look under DA lenses, etc.
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
In a nutshell
There are several series of Pentax K-mount lenses; the ones currently being produced include the DA (digital), DA L (cheap digital kit), D FA (full-frame, digital optimized), and FA (full-frame) series. While all Pentax K-mount lenses ever produced are compatible with all current Pentax DSLRs (in some cases with restrictions in that not all exposure modes of the camera are supported by old lenses), there are significant differences between each lens series and each has different features. Furthermore some of the newer lenses do not work with older cameras. We'll be exploring the differences between the various K-mount series lenses in this article. Read more about specific K-mount lens/camera compatibility here.
The DLSR designation below encompasses the mirror-less K-01.
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regarding vintage lenses and the 100mm f2.8 macro series:
" The optical design is the tried-and-proven formula of the previous generation D FA 100mm F2.8 Macro lens, which in turn had inherited the optical design from the well-respected FA 100mm F2.8 and F 100mm F2.8 macro lenses introduced in 1991 and 1987, respectively. None of these earlier lenses had rounded aperture blades. . . .
Read more at:
Pentax-D FA 100mm F2.8 WR Macro Review - Specifications | PentaxForums.com Reviews
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and the members of this forum is another nice reason to own pentax, very helpful and informative