Originally posted by hjoseph7 otherwise why confuse the heck out of people ?
The confusion comes as a result of the continued misguided effort to express FOV equivalence to the 35mm film format in terms of focal length. If degrees of arc were used instead, there would be no confusion. This is incredibly silly because the number of people in the current dSLR market have never used a 35mm film SLR and no basis for the comparison. Instead, there is marketing value in making people think they are getting longer lenses than what they actually paid for.
As for FF compatibility, there are two considerations:
- Mount. At present, all K-mount lenses are mount compatible with all K-mount cameras.*
- Image circle. This is the diameter of the projected image at the sensor plane. Lenses designed for small sensors have smaller image circles than those designed for larger sensors.
The bottom line is that any K-mount lens covering the 24x36mm frame will also cover the APS-C frame.
Steve
* Supported features may vary. There is also the issue of the Ricoh P variant of the K-mount which may be difficult to remove from a Pentax AF body.