On Sun, 28 Dec 2003 11:53:43 -0500, "matt"
wrote:


"William Hamblen" wrote in message
.. .
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.


I believe the OP question was _not_ answered. The way I understood it, he
was asking where is the _reference point_ from where the focal length is
considered for a multi-element eyepiece . For example let's say we have a 4
element ep , which are spread (due to their thickness and their spacing)
over a 1" length .Let's say the eyepiece is a 7mm FL. Where is the resultant
focal plane exactly ? Depending where is this "reference point" situated
lengthwise , one could have the focal place very close or very far from the
eyepiece end , or last/first element . This question is important if one
wants to know how to connect various accessories, tube length, focuser
length , etc . I'd like to see other answers than "just measure it" or "it's
hard to explain" , "do a google search" etc .

In eyepieces with an external field stop, the first principal plane is
one focal length away from the field stop. You could locate the
second principal plane by catching the image of the moon on a piece of
card stock and measuring one focal length in from that point.

To measure any better would take a well equipped optical shop. The
eyepiece supplier would have to provide any more information.