On Sun, 23 Dec 2001 21:49:29 GMT, d_ruether@hotmail.com (Neuman - Ruether) wrote:
>"Tony Morgan" wrote in
>message news:JWrHnUA8qbJ8EwxV@atomor.demon.co.uk...

[More on this, intended to suppliment earlier info
in this thread, as fever-dreams worked their way...;-]

>Varying standards of acceptable circles of confusion
>are necessary because of several factors which include
>optical characteristics due to focal length and degree of
>screen magnification, the size of the projected film, the
>projected distance, the magnification of the image
>and the viewing distance.

The above does not preclude other factors, a few of
which are:
- lens image-quality also affects DOF, with a less-sharp
lens having greater DOF than a sharper (a sharper lens
better defines the area of correct focus, leaving less
accurately-focused areas that would ordinarily be
included in the range of DOF looking noticeably less
sharp...)
- if the subject-plane is not parallel to the image plane
(not an uncommon lens fault...), DOF is affected with
the orientation of the lens...
- decentered lens element(s) can affect the apparent
DOF by sharpening one image edge relative to the
opposite one...
- field-curvature can affect DOF by altering the "plane"
of correct focus...
- imager discontinuities, as from "flapping" effects
and stair-stepping on scan lines in video, and any
other effects that produce bogus sharp edges (such as
oversharpening, common in video) tend to make the image
look sharper than it is...
- given an equal range of circles of confusion in front of
and behind the plane of focus, depending on the "bokeh"
characteristics of the lens (the difference in
contrast-rendering of front vs. rear out of focus areas
compared with the plane of focus), the DOF may appear
greater in front of, or behind, the plane of focus...
- the relative placement of size of the majority of detail
(in general, far subjects have more fine detail, less
well-rendered within DOF than the generally
larger-detailed near subjects), can cause distant parts
to look less sharp than near, all else being equal,
though both are within the predicted range of DOF...
- overall higher subject-contrast will tend to increase
the appearance of greater DOF than overall lower
contrast...
And to restate something surprising in the original
quotes: wide-angle lenses have less depth of focus than
long, all else being equal... Also surprising: all
lenses shot at the same stop have the same DOF, all
else being equal, whether WA or tele on a particular
format, with the images adjusted to show the same
area within the same final image size...