Originally posted by normhead you need a 600 FF on a 16 MP lens to fill the frame the same as a 400 mm on APS-c 16MP lens would. If you don't want to call it reach fine. What do you want to call it?
It's just called crop. You get no extra detail/IQ from the same lens put on an APS-C camera as opposed to a FF camera. There is just more periphery captured in a FF shot. And yes, of course, the same thing applies with a 645 vs a FF, and a 67 format vs a 645. The point is that FF is a good compromise between FoV and cost effectiveness for the general consumer. Plus it means we can use the FA limiteds straight onto the camera and get more frame to work with. A good reason to keep the FA Limiteds still selling...
Originally posted by normhead All that matters is on APS-c you get functionally the same image on a shorter lens.
No, man. That's only true if with the shorter lens on the APS-C you stand CLOSER to the subject than with the longer lens on a larger format camera. There is no extra REACH. If you stand at the same subject to camera distance and shoot with a shorter lens, you get a crop of the same FoV on an APS-C compared to a FF camera or any other larger camera.
See here:
Crop Sensor (APS-C) Cameras and Lens Confusion
plus you've been through this already in this thread:
https://www.pentaxforums.com/forums/photographic-technique/187306-aps-c-does-...ld-view-3.html
If you're still not convinced, see it diagramatically from Bob Atkins' article above:
and a practical example: