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