Originally posted by smcook99 I apologize, I'm must be slow today, I'm having a hard time picking up on what you're saying about K-mount lenses.
You're saying that all K-mount lenses will work on my camera but only shoot wide open. So lets rule out all the M42 for a second and run with that. You said my k20d is a K-mount. I just got a Tamron 28-75 f2.8 which fits my k20d and so I assume it's a K-mount lens. I know it doesn't shoot just wide open because it's in my hand. This leads me to believe I'm miss interpreting something about what a k-mount lens is. That is my first point of confusion. My 2nd point of confusion is... by 'only shoot wide open' does that mean if I get a 125 prime f4 that it will ALWAYS and ever be at f4, or does that mean it will only shoot at f4 if the camera tries to control it, or does that mean it will work the range of F stops if I mount it at a different aperture but never automatically stop down?
No. K is just the mount.
The old fully manual lenses (both exposure and focusing) like the Pentax Takumars in K mount will only shoot at the lens's widest aperture (smallest number) when used on your sophisticated DSLR, but you have to use a manual metering method in conjunction with your green button to get the correct shutter speed. (It's easy to do.) The M42 screw Takumars, however, allow you to use all of your f stops.
Newer auto metering lenses in K mount (whether autofocusing or not, like the Pentax M series and later) will operate fine and give you full automatic metering functions, just like a brand-new lens.