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[Phys-L] Re: optics terminology



Seems like another case of phrases having different meanings to different groups. Mike, could you suggest a better term for the definition all our testbooks are calling "focal point"?
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-----Original Message-----
From: Forum for Physics Educators [mailto:PHYS-L@list1.ucc.nau.edu]On
Behalf Of Edmiston, Mike
Sent: Tuesday, February 07, 2006 1:41 PM
To: PHYS-L@LISTS.NAU.EDU
Subject: Re: optics terminology


When I first responded to John's question, I was not in my office and
didn't have a supply of textbooks to examine. Now that I'm in my
office, I find the same thing John said. Every book I examined says the
focal point is the place where paraxial rays come to a focus. That
makes the focal length always equal to the distance between the lens and
the focal point, just like John said he read.

I am surprised I have just now noticed this, thanks to John, because I
do not agree with this. It appears to me that the textbook convention
is not consistent with everyday use of the word by practicing physicists
as well as the general public.

For example, the textbooks also state that the focal plane is one focal
length behind the lens, and that the focal point is where the focal
plane intersects the lens axis.

However, on a good camera the "focal plane" is marked, and it is the
film position. As we crank the lens out from its infinity position (in
order to focus on an object that is closer than infinity) the textbook
description would say that the focal plane moves out with the lens, and
moves to some place in front of the film. The textbook description
would place the focal plane ahead of the film even though the object is
actually in focus at the film plane, and were the film placed at the
textbook focal plane the picture would actually be out of focus.

I believe a photographer would say that the lens is adjusted to make the
focal plane coincide with the film plane. Of course, as the camera aims
out into the depth of the world, different planes in the world create
layers of different focal planes in the camera, and only one world plane
produces a focal plane coincident with the film plane at any one time.
But that's the way I think photographers view it. We adjust the lens
until the world plane of interest has its focal plane coincident with
the film plane.


Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu
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