@Meh - Exactly, it's the iris diaphragm that matters. Practically, since you have to actually do simple math (eek!) to determine that from the focal length and f/number, it's more practical to use those values to determine DoF.
LL's experiment with the gremlin is approximately correct. In fact, at the distances involved it's actually not the best approximation. I don't agree that the degree of unsharpness is identical - but it's close, good enough for approximation. I've tried an equivalent experiment, as a post hoc test, with my AFMA testing. Since I use 25x the focal length for all lenses, and the LensAlign fills the same proportion of the frame (i.e. distance and focal length are equal and opposite). With an actual distance scale on the ruler, it's apparent that f/2.8 at 16mm (16-35/2.8L II) and at 200mm f/2.8 (70-200mm f/2.8L IS II), the measured DoF is the same, when distance and focal length are reciprocal.
Actually, the reduction of DoF to dependence only on magnification holds at macro distances (and for microscopy, although we usually call it axial resolution not DoF, and use numerical apertures vs. f/numbers).
I thought I had this figured out till I read HillSilly's post...
Neuro or any other Savior : Please dumb this down for me... (you have done this many times
) : Bokeh notwithstanding.... do I get more or less OOF blur (Quantity) is APC-S or with FF.
Ok so let say i use a 50mm f1.8 on both a 5d and 7d, subject is 10 feet away from both cameras. After capture and 100% crop (lets say we frame 1 feet on all sides of the head of the subject (to get some background) in the crop )... basically like this frame for a passport picture
With image will show more blurring of the background?