Aperture setting for tube lens

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Aperture setting for tube lens

Post by billjanes1 »

Back when I was doing a lot of photomicroscopy, after setting up for Köhler illumination, I would remove the photomicroscope eyepiece and view the exit pupil of the objective (actually the Zeiss photomicroscope had an Optovar that could do this without removing the eyepiece). I would then close down the condenser diaphragm until about 80% of the exit pupil was illuminated. This was to establish a balance between resolution and contrast and was suggested by Zeiss.

I am now using a 200 mm f/4 vintage Nikkor as a tube lens for the Mitutoyo 10x objective. Looking from the rear of the tube lens, I could see the exit pupil of the objective. Closing down the aperture to about f/11 enabled the full exit pupil to still be visualized.

Does this make sense? Should I use f/11 on the tube lens?



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Post by Pau »

I think that this is not right.
At the microscope when you look without eyepiece or with a Bertrand lens you see the rear aperture, but the tube lens diaphragm is not placed at that optical position, you can close it to some extent until you get vignette, maybe F5.6 or F8, and image could see an small contrast improvement due to shading internal lens reflections (or no improvement depending of the lens): its role is closer although not fully equivalent to the microscope field diaphragm
If you place a diaphragm as close as possible to the rear of the objective it will act close to equivalent to the aperture condenser (or objective if bundled with) diaphragm of the microscope: more contrast and DOF and lower resolution.

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