Re: Optics question (not alt)

----- Original Message -----
From: "Jeffrey D. Mathias" <jeffrey.d.mathias@att.net>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Thursday, March 10, 2005 10:12 AM
Subject: Re: Optics question (not alt)

> Tom Ferguson wrote:
>> I'm wondering if Dan has confused the size of the image
>> versus the size of the circle of illumination (as a lens
>> is stopped down).
>
> Is this also know as the "circle of confusion" to those in
> the optical sciences?
>
> --
> Jeffrey D. Mathias
> http://home.att.net/~jeffrey.d.mathias/
>
>
  Different thing. What Tom is talking about is the increase
in coverage some lenses have when stopped down, somehing I
didn't think about when making my first reply. A lens
projects a circle of light called the circle of illumination
onto the focal plane. The quality of the image varies from
center to edge of this circle because some important
aberrations of lenses vary with image angle. They also vary
with the stop, so, the more the lens is stopped down the
sharper the image near the margins of the circle get. The
usable circle is called the circle of good definition. For
some lens designs the circle of illumination is very large
but the lens must be stopped down a lot to make it usable.
The well known Goerz Dagor is an example. A Dagor has a
circle of illumination of nearly 90 degrees. When wide open
a Dagor is sharp over perhaps 50 degrees. When stopped down
to f/45 it has a good quality image over nearly the entire
circle of illumination. Other lenses do not increase
significantly in coverage. The four element air spaced type
known as a dialyte or Celor has nearly its full coverage
when wide open. The size of the circle of good definition
increases very little as the lens is stopped down. Lenses
like the Dogmar, Apo-Artar, Apo-Ronar, and 70 Series Kodak
Anastigmat lenses are of this type. They have relatively
narrow coverage but are quite sharp wide open.
   Circle of confusion is the size of the blur spot a lens
produces from a point source object. It is generally used as
a criterion for the amount of defocus blur allowable for
visual sharpness in depth of field calculations. Depth of
field is based on the amount of blur the eye can tollerate
without seeing the image as blurred. All sorts of
assumptions are made about this which accounts for some of
the confusion about depth of field calculations.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com