Originally posted by Vpapad A series means original? Sorry for poor English..
The K-mount lenses evolved over time.
K and M are fully manual.
A are auto-aperture (the camera controls the aperture instead of using the manual ring)
F are Auto aperture and auto focus.
FA are a later automatic series, with some very nice lenses.
DA are built for use on digital bodies, with the smaller focus screen and sensor.
So if you have an M50mm you'll need to control everything manually. If you have an A50mm the your camera will control everything except focus.