[Phys-L] Re: optics terminology

[John Mallinckrodt <ajm@CSUPOMONA.EDU>]

> 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?

Maybe I'm missing something, but I've always understood that the
focal point was the point where the output rays from a lens (or their
backward extensions) converge IF those rays were input to the lens
parallel to its optic axis.  I've further understood that the focal
length is defined as the distance from the lens (or a suitable
principal plane) to the focal point.  Thus, the focal point IS "by
definition" one focal length away.

I guess I wouldn't be TOO surprised to find dictionaries defining it
the way you have suggested (as the distance to an in focus image or
object), but I thought the above was completely standard terminology
among physicists.  Can you give some examples of the definitions
you're referring to?

--
John "Slo" Mallinckrodt

Professor of Physics, Cal Poly Pomona
<http://www.csupomona.edu/~ajm>

and

Lead Guitarist, Out-Laws of Physics
<http://www.csupomona.edu/~hsleff/OoPs.html>
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