Originally posted by RuiC 645 lens with adapter to K will not crop on a FF camera. It will 1.5 on an APS-C camera !!
There is no
cropping going on here, APS-C is just a different format like APS-H, 4/3,35mm,645,67,4X5,5X7,8X10.
Vanya is correct, a 300mm 8X10 lens is still a 300mm lens when used on 35mm or APS-C, the only parameter that changes when you use a lens from a larger format on a smaller one is the FOV is reduced: this is a property of the format, not the lens.
So to answer your original question, the Pentax 645 SMC 120mm f/4 Macro on a 35mm sensor will have a focal length of 120mm, however it will have a narrower FOV than it would ordinarily have on the 645 format that it is designed for - end of story.