Originally posted by tripodquest But if the image circle covers a larger format without vignetting, wouldn't that mean that the image on a smaller format is heavily cropped?
Yes, that's why someone invented the term crop factor. And because of that, a 50mm on an APSc camera gives you the field of view of a 75mm on a FF camera.
What you throw away is 33% in each direction (36mmx24mm sensor/film size for FF/film versus approaximately 24mmx16mm sensor size for APSc). And hence the crop factor of 1.5x.
PS
It does not matter for which system that 50mm was designed; 50mm is 50mm. So if you put a DA50/1.8 on a film body, you will still get the field of view of an old 50mm on that same film body (possibly with severe vignetting).