So your subject's facial features taken with a 28mm lens on an APS-C will look identical to a picture taken with a 28mm lens on a full frame camera and identical to a picture taken with a 28mm lens taken with a medium format camera - taken from the same spot. The difference will be that the head in the shot taken with the APS-C sensor will take
"A fast lens. A stop is a stop; doesn't matter whether the lens is set up for full-frame or APS-C. APS-C vs full-frame concerns coverage area, not the inverse ratio of the aperture diameter to the focal length, which is how f-stops are determined (f-number = focal length / diameter of effective aperture).
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