One thing to remember about the mid-sized sensor-frame cameras is they place the lens farther away from the sensor than traditional SLRs do from the film. The effective result of that is a 1.5 multiplier to the focal length.
In other words, if you use a 50mm legacy lens, it will behave as if were a 75mm lens on a DSLR. So the 100mm macro lens that I shoot a lot of shots with on the K-3 is acting like a 150mm lens would on my K-1000.
Hope that makes sense. So if you want a regular "nifty fifty" to use as a prime, get a 35mm for that focal length (effectively 52.5mm).
