Originally posted by clackers Aperture, focal length and subject distance are variables, but not sensor size.
To replicate Field of View however (a different thing altogether), the larger format is forced to get very close indeed, and this may not be either practical (the subject says you're invading their personal space) or possible (an 85mm typically has a 90cm minimum focusing distance).
I kind of agree with this, but the problem with that is the change in perspective distortion by getting closer to the subject, so whilst you might achieve the same field of view on full frame by moving closer to the subject, you don't get the same photo (I know you and @Rondec know this - I'm highlighting it for others who might not
).
I hate to do this, but it brings us to the oft-discussed and well-worn topic of equivalence (rolled out here, again, for others' benefit):
When shooting with a 70mm lens at f/2.4 on APS-C, to achieve broadly the same photo on full frame, we'd need to shoot
from the same distance using a 105mm (70mm x 1.5) or thereabouts lens at ~f/3.5 (f/2.4 x 1.5). Simply using the same 70mm lens and moving closer will achieve a similar field of view, reduce the depth of field, but mess with the perspective distortion.
I know this is drifting off-topic a bit, but thought it worth re-stating