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[Phys-L] Re: optics terminology



Here we go again. I guess some people in order to avoid reading my
screeds don't read my posts, or was it too subtle?

Regarding the topic. One soln. is to use the term image, as in image
point and plane when one is not referring to a fixed characteristic of
the lens.

Also, alternately to ME's method, I was taught to use a nodal slide to
find the focal points of compound (thick) lenses.

bc, who has a set of thick lenses and a nodal slide in his museum.

p.s. It's supposed to be the only three worded paronomasia, and I
apologize in advance, as, perhaps, my post was simply overlooked, or
arrived after, the result of internet congestion.

John M Clement wrote:

Always remember the story about the 3 sons who started a ranch in TX
and asked their mother what to name it. She sent back a postcard with
the one word "focus". When they wrote to her again what it meant she
sent back a card with "That is where the sons raise meat".

John M. Clement
Houston, TX

PS: A perfect triple pun.




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"?
skip

-----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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