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From: John Denker <jsd@AV8N.COM>
Sent: Feb 7, 2006 9:31 AM
To: PHYS-L@LISTS.NAU.EDU
Subject: optics terminology
Hi --
I was recently forced to thinkg about the meaning of the term
"focal point".
I've always thought the focal point was where the image was in
focus, given a pointlike object ... or, reciprocally, the place
where the pointlike object was in focus, to produce a desired
image. Dictionaries seem to agree with me.
1a) As a corollary in the special case of a simple _astronomical_
objective, the focal point is one focal-length away from the lens
or mirror.
1b) Ditto for landscape photography (distant mountains).
2a) However, in the biology lab, where microscopes greatly outnumber
telescopes, the focal point might be nowhere near one focal-length
away from the objective.
2b) Ditto for macro photography (close-ups of bugs and flowers).
3c) Ditto for anything where multiple elements are involved
(e.g. catadioptrics among many other examples).
=================
The reason I bring this up is that I was flipping through a general
physics text that boldly claimed that the focal point was always
one focal-length away, by definition, without reservation. Yeccchh.
Anybody care to comment? Is there something I'm missing here?