Originally posted by stainsor Open aperture metering
stop down metering
aperture ring
screwdrive mount vs bayonet mount
Open-aperture metering: The aperture remains open when you press the shutter button to meter and expose. Common with some manual lenses.
Stop-down metering: The iris DOES stop down. Thus you can frame a shot with the aperture open, yet meter and expose at the desired aperture.
Aperture ring: Something not found on DA/digital lenses, where aperture is controlled from the camera. This ring allows you to select an aperture.
Screwdrive: The Pentax autofocus mechanism, only found on bayonet-mount lenses, those in the F, FA, and DA series. Not related to screwmount.
Screwmount: A threaded rather than quick-change lens mount. The vast majority of Pentax screwmount lenses use the M42 standard; a couple used M37. Leica Thread Mount (LTM) lenses use the L39 standard and can't be adapted to Pentax cameras. Many Russian lenses use the M39 standard, which CAN be adapted to Pentax. Other screwmounts exist but don't worry about them.
Bayonet mount: The quick-change mount type used by most modern SLRs. The Pentax design is called PK; it has evolved through several versions.
For more info, see Wikipedia and Google.