How important is axial exit/entrance pupil positioning in an afocal configuration?

[hans]

Assume perfect lateral and angular alignment between the camera and eyepiece but consider two extremes axially:

• Coincident: camera lens aperture set to match microscope exit pupil (or as small as possible, if it doesn't go small enough), exit/entrance pupils coincide axially.
• Maximum distance: camera lens set to maximum aperture and spaced as far as possible from the eyepiece without significant vignetting within the normal eyepiece FOV.

Has anyone done or seen comparison testing along these lines using an interchangeable-lens camera with good (but not necessarily too expensive) modern lens? With my current setup involving cheap camcorder tripod, bamboo skewers, and hot glue it is difficult to get them coincident, or even tell if that is actually possible, so I think most of the test photos I have taken so far are closer to the maximum distance case, probably with some lateral misalignment also.

The article by Charles Krebs "Two Basic Methods for Photomicrography" suggests they should be coincident but doesn't make it sound too critical: "The camera aperture should be set to the largest opening (lowest f-number)." Followed by some advice on positioning the camera, then: "Ideally, the camera will be positioned so that the Ramsden disk will fall inside the camera lens and coincide with the entrance pupil of the lens."

Theoretically it seems reasonable to assume some image degradation in the maximum distance case since more extreme parts of the camera lens are being used. On the other hand, the diameter of the beam contributing a particular image point is still limited by the relatively small exit pupil of the microscope which should limit many aberrations. It also seems like this type of modern fixed lens (like the Nikon DX 35mm 1:1.8G I have) outperforms plan achromatic microscope objectives by a fairly large margin, so maybe it is not too important to try to optimize the performance of the lens by making the exit/entrance pupils coincident?

[MichaelG.]

Honest answer: I have never done anything that would qualify as an experiment into this

Hypothesis: Except in the case of gross mis-match, the main effect will be that maximum light transmission occurs when the two pupils are co-incident.

MichaelG.

[abednego1995]

Good place to start would be the equations available at Edmund Optics. It's originally for determining the acceptable displacement between an infinite objective and it's tube lens but it also would apply in an afocal setup.

https://www.edmundoptics.com/knowledge- ... bjectives/

[Scarodactyl]

I did a vwry informal test holding my camera with lens over the eyepiece in my leotz trinoc port. Zoomed in on q cornerz carefully raising and lowering the camera had no visible effect on the image. Interestingly using the typical tests for this (finding the point where the iris causes even darkening, not vignetting) the lens shouls be very close to the eyepiece, but in the original Leitz setup it is significantly further up, so apparently it wasn't considered a big deal relative to being abke to put a shutter in between.

[BramHuntingNematodes]

Not with a modern lens. I have an afocal setup for my dynoptic because of the need for the compensating eyepiece. It's the 12.5 compens which has a 21 mm eyepoint, the highest of nearly all the eyepieces B&L had at the time including the "Hi-Point" eyepieces. For this setup, the two are about the same with an M12 lens as backing the camera back any more than 21 mm results in significant vignetting, maybe because the M12 lens is about the same size as a pupil. I find I get a clearer image by backing it up a bit anyway and using a telefoto lens, which helps. The outer 1/4 or so of the FOV gets cut out, but with the older, non-plan apos that's fine.

[patta]

It seemed to me that the differences are small or undetectable. As written above, it is probably due to the fact that the camera lenses has much less aberrations than the microscope objective.
Only theoretically:
- Matching the pupils (same position, same diameter) reduces the stray light
- The centre of the image is unaffected, since doesn't matter where is the pupil
- On the sides of the image, yes, we are using the worst part of the lens, so some aberrations will be stronger than with the centrally stopped down lens.
In the following link, they say that Coma is affected by wrongly placed pupil/aperture, page 37
https://www.iap.uni-jena.de/iapmedia/Le ... _112.pdf

So, at worst, a little more stray light, and at the image corner the camera lens has coma as at max aperture

[hans]

Thanks for all the input.

...the equations available at Edmund Optics...

So the equation being based only on vignetting at the maximum aperture of the tube lens, and the lack of mention of any other consideration, could imply the tube lenses are designed to work well all the way out to the maximum distance case, with no strong reason to prefer shorter distances? I suppose one issue comparing with camera lenses is that tube lenses could have been designed/optimized assuming this situation where the limiting aperture stop is in the objective and have somewhat different behavior vs. camera lenses optimized with the assumption that the entrance pupil is always filled with light in normal conditions?

I did a vwry informal test holding my camera with lens over the eyepiece in my leotz trinoc port. Zoomed in on q cornerz carefully raising and lowering the camera had no visible effect on the image.

Does your camera allow manual control of aperture in live view? I was finding this sort of test difficult not only because of the crude mechanical setup but also because the Nikon D5100 apparently only applies the current aperture setting to the lens briefly around shutter release. But then I read that the manual aperture setting actually is applied in live view, just only when initially entering live view, so you have to leave/enter live view mode in order to have changes take effect. I have not tried any further experimentation yet since learning about that quirk of operation.

Interestingly using the typical tests for this (finding the point where the iris causes even darkening, not vignetting) the lens shouls be very close to the eyepiece, but in the original Leitz setup it is significantly further up, so apparently it wasn't considered a big deal relative to being abke to put a shutter in between.

Does this mean Leitz made a camera lens specifically for afocal use with their standard eyepieces? I would be curious what the original setup looked like, which microscope?

...backing the camera back any more than 21 mm results in significant vignetting, maybe because the M12 lens is about the same size as a pupil.

Seems likely, I have similar experience when using a smartphone camera where I think the entrance pupil is generally ~1.5x the microscope exit pupil, depending on which objective, and assuming the numbers it puts in the EXIF data can be trusted.

In the following link, they say that Coma is affected by wrongly placed pupil/aperture...

Nice presentation, interesting to see so many different cases clearly drawn out and ray-traced, are you referring to page 37 "Effect of Stop Position"? That looks like a better/clearer way of thinking about what I was trying to say regarding "diameter of the beam contributing a particular image point is still limited by the relatively small exit pupil of the microscope" in my original post. So with the lens aperture fully open, going from normal photographic use where the stop is the lens aperture, to what I called "maximum distance" case where the stop is the microscope exit pupil, can be thought of in two parts:

• The reduced diameter of the stop has many beneficial effects familiar from normal photographic use.
• The shifted axial position of the stop has some detrimental effects as illustrated in the presentation.

It is not really obvious to me how things should end up overall. Intuitively it seems like going from coincident to maximum distance should cause no more degradation than going from stopped down to full open aperture in normal use, and possible much less degradation for some types of aberrations.

[hans]

Up one level from patta's link there is a whole course series of nicely-illustrated lecture slides, in case anyone is interested:
https://www.iap.uni-jena.de/iapmedia/Lecture/

[MichaelG.]

In the following link, they say that Coma is affected by wrongly placed pupil/aperture...

Nice presentation, interesting to see so many different cases clearly drawn out and ray-traced, are you referring to page 37 "Effect of Stop Position"?

Forgive me if I am being dim-witted, but in what sense does that page relate to the original question ?
... p37 appears to be about mis-location of the stop within the camera lens.

MichaelG.

[patta]

In the following link, they say that Coma is affected by wrongly placed pupil/aperture...

Nice presentation, interesting to see so many different cases clearly drawn out and ray-traced, are you referring to page 37 "Effect of Stop Position"?

Forgive me if I am being dim-witted, but in what sense does that page relate to the original question ?
... p37 appears to be about mis-location of the stop within the camera lens.

MichaelG.

Yes, at p.37, as Hans summarized:
the exit pupil, being smaller than the camera lens stop, acts as being the stop itself; so if exit and entrance pupil are in different places, it works as if the stop is misplaced (with respect to the design of the camera lens).
There aren't new aberrations, only the lens is stopped down in a less optimal way.

The exit pupil is a bit weird thing to figure out, it is a "stop" without physical iris: in fact it is the image of the physical stop located inside the microscope objective.

[hans]

(I see patta replied already while I was composing. This doesn't really add anything new but I will post anyways in case it helps clarify the relation to my original post.)

Forgive me if I am being dim-witted, but in what sense does that page relate to the original question ?
... p37 appears to be about mis-location of the stop within the camera lens.

Unless I am misunderstanding patta, it relates to the last paragraph in my original post, but not obviously, because I was still somewhat confused myself when I wrote that. In both cases I listed the diaphragm in the camera lens has no effect. (Except on stray light as patta mentioned.) So going from the coincident case to the maximum distance case would be equivalent to starting with normal photographic use having the lens stopped down to the same diameter as the microscope exit pupil then moving the axial position of the stop to the maximum distance position of the microscope exit pupil. In other words, is is possible to think of the maximum distance case as a "mis-location of the stop within the camera lens" as you put it.

[patta]

we are rapidly converging somewhere

[Scarodactyl]

Does this mean Leitz made a camera lens specifically for afocal use with their standard eyepieces? I would be curious what the original setup looked like, which microscope?

Correct. This is how Leitz and Zeiss did many or most of their late finite camera setups. The lenses were just plain flat achromats. A modern pancake seems to work as well or better.

[patta]

Here a drawing about eyepiece exit pupil (objective aperture stop image) vs lens entrance pupil (lens aperture stop).
Was attached to the previou post, may be useful to understand what speaking about.

[MichaelG.]

Does this mean Leitz made a camera lens specifically for afocal use with their standard eyepieces? I would be curious what the original setup looked like, which microscope?

Correct. This is how Leitz and Zeiss did many or most of their late finite camera setups. The lenses were just plain flat achromats. A modern pancake seems to work as well or better.

For what it’s worth ... here’s a discussion that I started about the Zeiss one:
viewtopic.php?f=5&t=7871&hilit=afocal

MichaelG.

.

P.S. __ Hans : This is a very good primer, with some excellent images [fig.1] of actual exit pupils:
https://www.quekett.org/wp-content/uplo ... camera.pdf
What Peter doesn’t know about this stuff is probably not worth knowing.