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
> 
>