Originally posted by Porffor Rare to find a set of actual PENTAX branded tubes.
Universal M42 screw fitting with auto iris pins in original case
Arg. M42 is the previous mount for Pentax (before 1975!). So you won't be able to mount it on your body unless you also add an M42 adapter (on the body), and use it with M42 lenses (lenses Takumar).
M42 is a Pentax mount but was used by most manufacturers on that era. M42 is mostly a "dumb" mount, there is no connection between body and lens.
What you need to be careful before buying something for your Pentax body is something "K mount". But on that, you are lucky, because almost every lens K mount will mount and work on your body.
Here are the different K-mount (lens side) :
K : the original mount, without electrical contact (Pentax K and Pentax M lenses) - all will work on your K100D but metering can be annoying
KA : ability to select aperture on the body (A position on the aperture ring) - all will work on your K100D without limitation
KAF : autofocus with motor in the body (with or without aperture ring) - all will work on your K100D without limitation
KAF3 : autofocus with motor in the lens - most will work on the K100D but autofocus will not work - DA(L) 18-50 RE won't work because manual focus is driven by the motor (focus-by-wire)
KAF4 : electronic aperture - won't work on the K100D (for example the DA 55-300 PLM)
Stay far from KR lenses, they can mount but will get stuck. They can be recognized with "KR" on the aperture ring
and P position on the aperture ring (as opposed to A position, which is fine).
That said, M42 lenses are often coveted. But you will need a K-to-M42 adapter for these. Genuine Pentax are by far the best, but they are not available new. M42 lens with adapter will work on your K100D without limitation (better than the Pentax K and M series since there will be no metering limitation).