@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).
Neuro: 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. Give me some numbers in feet etc if you can... cause "framing and similar terms are relative
I'm no neuro but I can take a shot to rephrase it into less technical terms.... we'll see if I have this right!
If you use the same lens, set to the same aperture, and place the camera the same distance from the subject, you will get more OOF blur with an APS-C sensor compared to a FF sensor.
If you use the same lens, set to the same aperture, and place the APS-C camera 1.6X further away from the subject than the FF camera (to get the same framing) you will get more OOF blur with the FF camera.
If you use the same lens, set to the same aperture, and place the APS-C camera 1.26X further away from the subject than the FF camera you will get the same OOF blur. (1.26 is the square root of the ratio of CoC of FF and APS-C)
If you use different lenses (two different focal lengths) on the same camera but set to the same aperture, and place the camera at the same distance from the subject, the shorter focal length will give less OOF blur.
If you use different lenses (two different focal lengths) on the same camera but set to the same aperture, and place each camera at 25X (or any multiple) of the focal length, both lenses will give the same OOF blur. (This is the gremlin example on LL that keeps the subject the same size)