I personally make a distinction between the "focal length" and the
"focal point." I don't think I am alone in making this distinction, but
I haven't conducted a poll.
The focal length is the distance from lens to focal point when the
entering light is parallel to itself (i.e. not diverging or converging,
i.e. coming from infinity) and also parallel to the optical axis of the
lens. If the incoming light is not parallel, then the focal point will
not be one focal length from the lens.
Stated another way, the focal length of the lens is fixed (excluding
zoom lenses) but the focal point is not fixed. The focal point wanders
"in and out" if the incoming light changes in convergence/divergence.
The focal point (or the image) of a point object wanders around the
focal "plane" if the point object wanders off axis. The focal "plane"
is not necessarily a plane, i.e. not necessarily flat.
Of course with compound lenses the location of the lens is ambiguous. I
am aware of several ways of dealing with this. The least desirable, I
think, is to measure from the center of the compound lens. I believe a
more meaningful focal length for a compound lens results from looking at
the solid angle formed using the focal point of parallel light as the
apex, then looking at the total cone of light coming into that point.
Extending the cone height until the base reaches the diameter of the
entrance lens yields an effective focal length, and that height divided
by the lens diameter is the effective f-ratio. I think this is the way
camera lenses are figured, and I think this is the way compound
telescopes such as Schmidt-Cassegrain and Maksutov-Cassegrain are
figured.
I am no expert, particularly in microscopes, but I have dabbled a lot in
photography (especially 35mm) and also in telescope building. The ideas
I've presented here are consistent with what I have heard and have read
in these areas.
Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu
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