[Phys-L] Re: optics terminology

[John Mallinckrodt <ajm@CSUPOMONA.EDU>]

>  >From the Photonics Dictionary:
> <http://www.photonics.com/dictionary/>
>
> Focal length: The effective focal length (EFL) is the distance from the
> principal point to the focal point. The back focal length (BFL) is the
> distance from the vertex of the last lens to the second focal point. The
> front focal length (FFL) is the distance from the first lens surface to
> the first focal point.
>
> Focal plane: A plane (through the focal point) at right angles to the
> principal axis of a lens or mirror; that surface on which the best image
> is formed.
>
> Focal point: That point on the optical axis of a lens, to which an
> incident bundle of parallel light rays will converge.
> ----------------------
> The Photonics Handbook is another useful guide for practitioners of
> optics.
> <http://www.photonics.com/directory/hb/XQ/ASP/QX/index.htm>

Perhaps, but I think the above is not a very good advertisement for
the utility of the guide,

The definition of "Focal point" seems to be TRYING to agree with my
understanding of the term, but it is inexcusably sloppy as it implies
incorrectly a) that there is only one focal point, b) that "incident
bundles of parallel light rays" are forced to converge by a lens, and
c) that "bundles of parallel light rays" necessarily meet at a "point
on the optical axis."

I assume they mean to say something MORE like "A point on the optical
axis of a lens, to (OR FROM) which an incident bundle of LIGHT RAYS
PARALLEL TO THE OPTIC AXIS will converge (OR APPEAR TO DIVERGE)."

Given that definition of "Focal point" the definition of "Focal plane
makes absolutely NO sense to me.  The "surface on which the best
image is formed" simply does NOT in general include the "Focal point"
as defined here.

--
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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