Originally posted by timo DOF is simply a measure of what looks acceptably sharp. It is entirely a matter of perception. DOF is not an absolute.
Quite correct, but incomplete. I phrase it as:
DOF is a complex product of photography, presentation, and perception. We wage the DOF WARS, arguing over our attempts to simplify the complexity, leaving out crucial factors.
Photography: Frame size, focal length, aperture, subject distance, context (surroundings), even light, are factors the photographer can more-or-less control when shooting.
Presentation: Enlargement size, mounting and displaying (media, placement, light, etc), viewing distance, context, are factors the photographer might not be able to control.
Perception: Acceptable sharpness, visual acuity, viewers' attention spans and mental states and visual literacy and biases, are factors the photographer can't control at all.
We try to simplify this, by controlling certain factors and saying, ALL ELSE BEING EQUAL -- but all else AIN'T equal. We shoot, present, and view images, all in varying conditions. We may try to carefully control DOF in a shot. But if the subject is some boring crap, potential viewers won't even bother looking, at least won't pay close enough attention to see those finely-crafted details. Bother. They'll also neglect the picture if they need to piss, or to tend to other real-world stuff.
Acceptable sharpness loses its meaning then. Bother.
ObTopic: The question was answered. On a crop dSLR, adjust a FF lens by just over 1 f-stop, and don't adjust DA lenses. And now, back to the DOF WARS!