Originally posted by nessamento i don't know much about lens, what exactly does it mean if it's an M lens?
When the K2 and K1000 were introduced in 1976, Pentax also introduced the k-mount bayonet lens mounting system. Many of the lenses introduced at that time, and for several years after, were reworked Super-Takumar and SMC-Takumar, screw-mount lenses. They were the same optically and the same size, as well.
Then, sometime around 1980, Pentax, in response to the success of the Olympus OM series cameras, introduced the M-series cameras, which were much smaller and lighter than the first-generation k-mount cameras. A new series of lenses was introduced at the same time, the M-series. The M-series lenses had no major new features, there were just in smaller packages, to match the smaller cameras.
What many people call K lenses (Canada Rockies is correct about the true nomenclature) and the M-series lenses are pretty much interchangeable, with no loss of features. That is, an original PK lens can be used on an M-series camera, or an M-series lens can be used on a K-1000, with equal funtionality. In fact, the K-1000 lasted so long that it was delivered, over the years, with whatever Pentax lens was then current, whether it was PK, M-series or the later A-series.
After the M-series lenses, came the A-series, which first appeared on cameras like the Program Plus and AutoProgram. These lenses have electrical contacts for the lens and camera to better communicate, especially aperture information.
Pentax likes to brag that ANY lens ever made for a Pentax 35mm camera can be used on their new digital cameras. This appears to be true. I use the 50mm, f/1.4 Super-Tak that came on my Spotmatic, on my K10D and it works perfectly. Of course, I don't get autofocus or autoexposure, but I do get nice, sharp pictures. I just set the camera in Av mode.