Originally posted by jatrax I will take a try at explaining this though I'm sure someone with a better grasp of the technical details will also reply. To answer your question: NO, if you buy a 35mm lens you have a 35mm lens. The crop factor has nothing to do with the focal length. The difference between a 35mm lens designed only for APS-C and a 35mm lens that is designed for a FF sensor is that the APS-C only lens will not cover the entire area of a FF sensor. If you are going back and forth from film to APS-C sensor then there is a difference in how things will look through that 35mm lens, and that is due to the crop factor. If you only shoot APS-C then forget you ever heard of 'crop factor', just look through the view finder and shoot what you see.
So if you are intending to shoot only APS-C, buy any lens you want, as long as it says Pentax on it , it will work. If you are worrying about a future FF camera and want your lenses to work there as well things are a lot harder. As noted above some Pentax lenses do work on FF but until Pentax releases such a camera we are only speculating which ones will work correctly.
Jatrax is kind of right. The focal length of a lens is property of the lens. A 28mm lens is always a 28mm lens no matter what camera you put it on.
The crop factor gives an indication of how much the sensor size chops off relative to 35mm film camera. The changes the effective field of view; the crop factor lets you calculate the focal length required on a 35mm film camera to get the field of view you are observing with your camera and lens (and calculate back again).
Example: that 28mm lens on an aps-c camera gives the same field of view as a 42mm lens on... The picture is framed the same. But, the 28mm lens is still a 28mm lens.
If could fit all the lenses in the world on a single camera then all the lenses with the same focal range would give the same field of view (colours, contrast, distortions etc would vary greatly!). But if you took one lens and put it on all the cameras in the world you'd get different field of views grouped on camera sensor size!
Should you forget about crop factor, as Jatrax suggests? I don't think so, using the crop factor to calculate equivalent field of view on a 35mm film camera is useful when discussing photography and is often used in learning resources, as they are so many different sensor sizes out there!
As for FF vs APS-c lenses. You need the lens to be able to project an image to cover the sensor... FF sensors are bigger than APS-c. Therefore there is no problem using a FF lens on an APS-C camera, but there is likely to be trying it the other way round!