Posted by
Guy Cox on
URL: http://confocal-microscopy-list.275.s1.nabble.com/objective-focal-length-tp1588271p1588540.html
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
Optical Imaging Techniques in Cell Biology
by Guy Cox CRC Press / Taylor & Francis
http://www.guycox.com/optical.htm______________________________________________
Associate Professor Guy Cox, MA, DPhil(Oxon)
Electron Microscope Unit, Madsen Building F09,
University of Sydney, NSW 2006
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http://www.guycox.net-----Original Message-----
From: Confocal Microscopy List [mailto:
[hidden email]] On Behalf Of Aryeh Weiss
Sent: Friday, 28 November 2008 7:20 PM
To:
[hidden email]
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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