On 27 Dec 2003 19:03:29 -0800, (David Neal
Minnick) wrote:

Can someone draw me a word-picture or direct me to a resource showing
just how the focal length of an ep is measured? That is, if one could
cut an ep in half lengthwise showing everything intact, from which
part to which part is the f/l measured?

If you know the curvatures, thickness and type of glass for each
element in the eyepiece you can calculate the focal length of the
eyepiece. That's what lens designers do. If you had an optical bench
you could measure it directly. There are natural variations in
manufacturing tolerances so the focal length engraved on the eyepiece
may not be exact. Or the design can change, but the markings stay the
same.

Lenses have imaginary planes called the principal planes where the
rays of light that enter and exit seem to bend when you trace the
rays. The locations for any given eyepiece depends on the design.
The focal length is measured from those planes.