RE: Image sharpness and focus distance

["Xiong, Zonghou" <Zonghou.Xiong@xxxxxxxxxxxx>]

Regarding "circle of confusion" does it have to be determined
arbitrarily or can it be defined by the size of film grain?

I'm not sure how lens manufactures define this term and use it
to compute their DOF marks on a lens.  Obviously if an dot
is rendered smaller than the film grain it is within the 
absolute DOF of the film, regardless size of enlargement. Then
again, different film have different DOF on a lens.

Zonghou Xiong

-----Original Message-----
From: Andy Buck [mailto:buckwiet@xxxxxxxxx]
Sent: Wednesday, October 25, 2000 11:51 PM
To: panorama-l@xxxxxxxxxxxxxxxxx
Subject: RE: Image sharpness and focus distance


The following is cut&pasted from a FAQ on
www.graflex.org (a great site for lens info):

"When a lens is defocused, a point in the subject gets
rendered as a small circle, called the circle of
confusion. If the circle of confusion is small enough,
the image will look sharp. There is no one circle
"small enough" for all circumstances, but rather it
depends on how much the image will be enlarged, the
quality of the rest of the system, and even the
subject. Nevertheless, for 35mm work c=.03mm is
generally agreed on as the diameter of the acceptable
circle of confusion. Another rule of thumb is c=1/1730
of the diagonal of the frame, which comes to .025mm
for 35mm film. (Zeiss and Sinar are known to be
consistent with this rule.)"

Based on all the responses so far, the areas in a
panoramic image that the lens is not exactly focused
on are sharp, fuzzy, blurred, out of focus, and
stretched or compressed, all depending upon the viewer
and the subject: phew! <g>

Andy


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