Hi Aryeh

Maybe the tube lens is not the original one, so that the "25x" 
objective really functions as a 10x in this setup?
Can you give us more info about the setup. A 25x/0.5 air objective 
with a 11mm WD must be pretty big...

Beat

At 09:20 28-11-2008, you wrote:
>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