Originally posted by Joshua A I might of possibly made some errors when it comes to calculations, but as long as the incident angle on the sensor is equal between sensors, wouldn't that mean that the field of view between the two cameras would be equivalent.
Specific types of lenses - such as Pentax K-Mount and Asahi Takumar - are designed to fit one, single registration distance. In our case 45.46mm (
Flange focal distance - Wikipedia, the free encyclopedia ). Mounting such on a slimmer body with a smaller mount-sensor distance while still achieving focus will require either:
A. an adpater (hollow, "passive" tube) that adds to the lens mount-sensor distance. It does not change the focal lenght, but the smaller the sensor, the more you will crop the image that the lens produces. You will not retain the FOV in that case.
B. a telecompressor/focal reducer that reduces the focal length of the lens and reduces the registration distance, not for the lens itself but for the lens+telecompressor combination as a whole. Within certain physical limits you might design a telecompressor that would give you about the same FOV on a smaller sensor.
Designing new lenses with a smaller registration distance for smaller sensors (and most likely also smaller image circle to reduce dimensions and weight) is "easily" done,
but making such with K-mount and calling them K-mount lenses would create havoc......