Re: [Leica] Depth of field summary with regard to the argument at hand

[John Collier <jbcollier@home.com>]

Aha! Now we are down to the brass tacks!

Perceived depth of field varies with the magnification of the subject
(combination of output size and viewing distance), not with the actual
physical size of the subject. If a picture is taken of my son and I,
assuming it is competently done, the extent of the perceived depth of field
in front and behind us will depend on the physical size of the aperture and
the magnification in the viewed final output, not our differing height and
masses.

Depth of field calculations assume a fixed output size and viewing distance.
This can be a negative, positive, print or projection and seems to be
expressed as having the viewing distance set as a ratio of the diagonal of
the final output (not quite sure of the specifics) The variables are:

1) the physical size of the aperture, which affects the rate at which the
circles of confusion get larger as you move away from the plane of focus.

2) the focused distance, which affects the magnification of the subject.

3) the focal length of the lens, which also affects the magnification of the
subject.

4) the film format, which also affects the magnification of the subject.

If you do not standardise you final output, then anything goes.

John Collier

> From: "austin@darkroom.com" <austin@darkroom.com>
> 
> I am curious about something, regarding the 'print DOF'.  Won't objects that
> are 'larger' 'appear' to be sharper than smaller objects?  If so, then
> shouldn't the DOF calculations have a stated object size/distance (to other
> objects) to actually make these DOF 'tables' more 'accurate' (what ever
> accurate is in this case)?