The paper says it is a Zeiss Planachromat 25x/0.5 objective. I 
searched for such an objective and the only I found was the Zeiss 
Jena GF-Planachromat shown at
http://cgi.ebay.de/Zeiss-Jena-GF-Planachromat-Phv-25x%2F0,50-Phako--neu-_W0QQitemZ380085053453QQcmdZViewItemQQimsxZ20081127?IMSfp=TL0811271110001r39141

It's easy to see by the diameter of the front lens (<5mm) alone that 
this objective cannot possibly have a working distance of 11m at an 
NA of 0.5. So it seems very likely that the specs given in the paper 
are simply wrong.

Beat



At 18:39 29-11-2008, Aryeh Weiss wrote:
>Content-Transfer-Encoding: 7bit
>
>The problem is that an infinity corrected 25x lens with a 25mm focal 
>length should require a tube lens with a 625mm focal length (25x25). 
>Does such a microscope exist?
>
>--aryeh
>
>
>Patrick Van Oostveldt wrote:
>>Dear Arey,
>>Not being an expert I should say: if the tube lens forms an image 
>>at 245mm (=focal length) and if the lens magnification is 25x the 
>>objective should have a focal length of 9.8mm. The working distance 
>>is smaller than this, but is not directly predictable.
>>However the magnifcation inscribed on the lens (25X) is dependent 
>>on a specific focal length of the tube lens. For Nikon objectives 
>>it is 200mm for Zeiss it is 164.5nmm. Probably this can be the 
>>point of confusion.
>>Best regards
>>Patrick
>>Quoting Aryeh Weiss <[log in to unmask]>:
>>
>>>That is also how I understand it. So I assume that the distance between
>>>the tube
>>>lens and the intermediate image plane is the focal length of the tube lens.
>>>
>>>Best regards,
>>>--aryeh
>>>
>>>Guy Cox wrote:
>>>>I am absolutely not an optics guru!
>>>>
>>>>But, as I understand it, an infinity corrected objective will form an image
>>>>at 'infinity', so the distance from the principal plane to the
>>>>object will be
>>>>the focal length.  But where the image is formed after the tube lens will
>>>>depend on the focal length of the tube lens and we have to know 
>>>>this to make
>>>>any useful calculation.
>>>>
>>>>It was all so much simpler in the days of 160mm tube length!
>>>>
>>>>Guy
>>>>
>>>>-----Original Message----- From: Confocal Microscopy List
>>>>[mailto:[log in to unmask]] On Behalf Of Aryeh Weiss Sent:
>>>>Friday, 28 November 2008 7:20 PM To:
>>>>[log in to unmask] Subject: objective focal length
>>>>
>>>>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
>>>>
>>
>
>
>--
>Aryeh Weiss
>School of Engineering
>Bar Ilan University
>Ramat Gan 52900 Israel
>
>Ph:  972-3-5317638
>FAX: 972-3-7384050