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
>>
>>



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