I am confused with regard to the front focal length of objectives. I thought 
that the magnification of an infinity corrected objective will be the distance 
between the tube lens and the intermediate image, divided by the front focal 
length of the objective (which is where I expect the object to be). Since the 
objective is not really a thin lens, I can understand that the actual working 
distance may be less than the focal length, since the focal length may need to 
be measured from inside the objective casing.

However, I have a paper that describes a 25x/NA=0.5 air objective, which has an 
11mm working distance, as having a 25.1mm focal length, while the distance from 
the tube lens to the intermediate image is 245mm. This has me confused, and I 
realize that I do not understand something fundamental here.

So, I turn to the optics gurus on the list to clear this up, with many thanks in 
advance.

--aryeh
-- 
Aryeh Weiss
School of Engineering
Bar Ilan University
Ramat Gan 52900 Israel

Ph:  972-3-5317638
FAX: 972-3-7384050