On Sun, 23 Dec 2001 20:38:22 GMT, "David McCall" wrote:
>"Tony Morgan" wrote in
>message news:JWrHnUA8qbJ8EwxV@atomor.demon.co.uk...

>> Don't know if this is true since (all other things being equal) the
>> circle of confusion can only be the physical size of a single pixel.

>> >The main thing that effects depth of field is aperature size.

>> AFAIK the focal length has just as important influence.

>I believe this to be over simplified.
>
>Acording to the American Cinematographer's Manual (1969);
>
>"The extent of the depth is dependant upon the permissible
>blur which can be accepted as sharp. This acceptable
>point or circle, is refered to as the circle of confusion."
>
>"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."
>
>"Depth of field depends on the f/stop, the focal length of
>the lens, the circle of confusion, and on the distance of
>the lens from the subject."
>
>So yes, focal length does affect the depth of field, among
>other factors, but it is a constant that is defined by the
>needs of the shot. The variable that affects depth of field
>is the f/stop, which can be altered through the use of ND
>filters, lighting, emulsion sensitivity (gain in the case of video).
>
>While grain (pixel size in video) does contribute, I don't
>think it alone defines the acceptable focus or the size of
>the circle of confusion.
>
>The following is extracted from the 16mm tables in that
>book. In video terms this might be similar to 2/3" chips.
>Chip size is much smaller than this on the cheaper cameras.
>
>Since I can't do a chart very well without HTML,
>(all are with the camera focused at 4' with the
>circle of confusion assumed to be .001")
>I'll use the following format ;
>Focal length, f2= near/far, f22= near/far
>
>10mm, f2= 3' 6"/infinity, f22= 6"/infinity
>16mm, f2= 3'3"/5' 3", f22= 1' 2"/infinity
>25mm, f2= 3' 8"/4'/5", f22= 1' 11"/infinity
>35mm, f2= 3' 10"/4' 2", f22= 2' 7"/8' 6"
>50mm, f2= 3' 11"/4' 1", f22= 3' 2"/5' 5"
>75mm, f2= 4' / 4' 1", f22= 3'7"/4' 6"
>100mm, f2= 4' /4', f22= 3' 9"/4' 3"
>
>Just as a side note, the original poster was asking about
>depth of focus, and we all assumed that he ment to say
>depth of field. Depth of focus is a real measurement too.
>again from the American Cinematographer's Manual (1969);
>
>"The depth of focus is an infinitely small range behind the lens
>at the focal plane within which the film is positioned durring
>exposure. This is most critical, particularly withj short focus
>lenses. If the film moves out of this precise position, either
>forward or backward, it will cause unsharp images produced
>by an increase of the diameter of the circle of confusion."

All this is great information, and correct as far as it
goes, but a few notes on the following may be
interesting...:
"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...
- having subject-plane not parallel to the image plane,
and decentered lens element(s) can affect the apparent
DOF...
- field-curvature can affect DOF...
- imager discontinuities, as from "flapping" effects
and stair-stepping on scan lines, and any other effects
that produce bogus sharp edges tend to make the image
look sharper than it is... Along with this are image
oversharpening effects...
- given 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), and also of relative
placement of size of the majority of detail (far subjects
have more fine detail, less well rendered within DOF
than the generally larger-detailed near subjects),
causing distant parts to look less sharp than near, all
else equal, though both are equally 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...
(I trust a cold permitted me to be coherent in the
above...;-)