Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

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hans
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Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#1 Post by hans » Thu Mar 25, 2021 2:26 am

When a condenser is described as aplanatic/achromatic or whatever, what is the conjugate pair of image planes being considered? Infinite-conjugate imaging of the condenser front focal plane onto the objective side? Imaging of the field iris into the specimen plane? Something else?

I have seen this article and linked it a few times in previous discussions:
Microscopy Primer by Frithjof A. S. Sterrenburg - 7. ILLUMINATION IN MICROSCOPY
However it doesn't actually explain much at a fundamental level and after re-reading it a few times I doubt whether certain parts are correct, so another source would be nice.

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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#2 Post by apochronaut » Thu Mar 25, 2021 8:49 pm

The front focal plane of the condenser is conjugate with the back focal plane of the objective. The back focal plane of the condenser is conjugate with the specimen plane.
The N.A. of the cone of light that is free from spherical aberration in simple condensers ( those up to abbe design parameters) is very narrow, much more narrow than the stated N.A. of the condenser, about 1/2. Thus aspheric or aplanatic condensers are capable of passing a broader cone of focused illumination to the specimen, closer to their stated N.A. and thus provide higher N.A. objectives with illumination of a higher N.A. and resolution potential.

While simple condensers would therefore seem to be as good for lower N.A. objectives, say up to about .50 or so, the broader field coverage required requires a wider angle condenser and thus lateral ca becomes an important negative consideration. Thus, an achromat condenser with maximum correction for lateral ca is superior to an aplanat for low N.A. maximum resolution performance, with achromat/aplanats performing best over a broad range of N.A. requirements, closely followed by abbe aspheric types. High N.A. abbe types( 1.4 N.A.) are probably next best for high N.A. objectives while lower N.A. achromats would be best for lower N.A. objectives of superior colour correction. W.F. abbe objectives would do well for low N.A. objectives too. I only know of one that I would classify as a W.F. abbe aspheric and it is in fact an excellent condenser covering a broad range of requirements with excellent performance.

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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#3 Post by hans » Thu Mar 25, 2021 11:05 pm

So in the basic brightfield Köhler configuration which image is aplanatic/achromatic? The image of the condenser iris projected at infinity or the image of the field iris in the specimen plane? I am thinking they cannot both be corrected simultaneously especially in relatively simple designs like the Reichert 1970 or 1201. (Similar to how an objective is corrected at a specific working distance/magnification and imaging degrades if used away from that.)

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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#4 Post by apochronaut » Fri Mar 26, 2021 12:37 pm

I'm not sure why not both? Except that the image at the back focal plane of the objective has been created by a relay system of lenses( the objective).

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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#5 Post by hans » Mon Mar 29, 2021 8:38 pm

I'm not sure either. My vague understanding so far is that lens systems generally have a single pair of conjugate aplanatic points where the sine condition is satisfied. Not clear to me whether it is theoretically possible to have multiple pairs of aplanatic points or how much complexity that would add, but for example seems unlikely that adding a single aspheric surface to the basic Abbe design (which I think is what the various AO/Reichert "Abbe aspheric" designs do?) would result in more than a single pair of aplanatic points?

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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#6 Post by apochronaut » Tue Mar 30, 2021 3:10 pm

It is easy to feel that a single change in one lens surface would do little to alter the characteristics of the entire lens system but the goal of the aspheric surface would be to correct for a certain condition, that of spherical aberration, and thus produce a condenser that could provide a well focused illumination plane , better over a wider field than an abbe.There is no claim that the condenser is a perfect condenser, just more highly corrected than an abbe. The fact that AO/Reichert offered that condenser as a default standard condenser was a bit of a gift really, when many competitors offered only an abbe. It is wide field too, easily filling the field of a 4X objective at a 20mm f.o.v.
AO/Reichert did also produce a 4 element 1.4 N.A. achromat aplanat, corrected also for ca, with a wider N.A. free of s.a.., so higher resolution objectives could benefit from it's performance, across a wider field.

In use, if one is using an objective of 1.3 N.A. or higher, there is a considerable difference in the resolution achievable when using the 1.4 condenser. I don't use the 1.25 abbe aspheric with 1.30 planfluor or 1.32 planapo objectives but prefer it with the lower magnification objectives 2X planfluor, 2.5X planachro and 4X planachro, where it's wide field characteristics yield a broad well lit even f.o.v. It is also perfectly good with planachro and planfluor 10X, 20X and 40X , interchangeable with the .90 achromat aplanat for those objectives.

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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#7 Post by hans » Tue Mar 30, 2021 8:55 pm

Agree the purpose of the aspheric surface is to correct spherical aberration and no doubt it achieves that to some degree for at least one specific working distance. I doubt whether spherical aberration can be corrected for more than one working distance by adding a single aspheric surface.

With objectives the corrections are effective at a specific working distance/tube length/magnification and degrade quickly when deviating from that at high NA, and I would assume a condenser behaves similarly?

Also consider the case of a single spherical lens bent to minimize SA when focusing collimated rays from one side. The bending is not optimal to image collimated rays from the other side -- would need to be reversed. This is analogous to what I am wondering about condensers. If the condenser is optimized to focus collimated rays from the bottom with minimal SA (the case of imaging of the field iris into the specimen plane) it seems unlikely it would simultaneously achieve a similar level of correction for collimated rays on the top (the case of imaging the condenser iris to infinity). If the two cases are corrected simultaneously to a similar degree it would be nice to find a source explaining how exactly that works. If not, then which case is optimized?

The stuff you mention about how condenser NA/correction affects the final image is interesting, and what I was hoping to understand eventually. But in the process realized I was not even sure about this much more basic question of which images locations are corrected.

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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#8 Post by apochronaut » Tue Mar 30, 2021 9:30 pm

Is the image of the condenser iris not further corrected in the objective, though? Incidentally, Milton Sussman patented two abbe aspheric condensers within a year of each other. One is a two element and the other a 4 element in 2 groups.

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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#9 Post by hans » Wed Mar 31, 2021 2:30 am

Yeah, the objective complicates things, and perhaps it does not make much sense to try to understand the condenser corrections in isolation. Intuitively it seem to me that combined imaging behavior from condenser FFP to objective BFP would be the most important place condenser aberrations have a role, because the sizing of the condenser iris relative to the objective aperture has a much larger effect on the image in most cases than the field iris, and also for contrast techniques phase, DIC, etc.

Maybe the situation could be -- condenser and objective both optimized for imaging in the specimen plane, both image poorly FFP/BFP to infinity in between, but those aberrations tend to cancel well when considering combined imaging FFP to BFP?

Thanks for the patent pointers, hadn't seen those, and looks like they may have some good hints.

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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#10 Post by hans » Mon Jul 26, 2021 4:18 am

GB1162911A - Aplanatically Corrected Condenser, some excerpts:
The conventional illumination principle of Koehler operates with an optical illumination system which substantially comprises two optical elements, viz. a collector and a condenser, and two mechanical diaphragms, viz. a bright-field diaphragm near the collector and an aperture diaphragm containing the front, i.e. image-side, focus of the condenser. The collector images the light source in the aperture diaphragm, while the condenser images the bright-field diaphragm in the object plane and the aperture diaphragm at infinity.
Frequently, the hitherto known condensers are not corrected either chromatically or aplanatically, which is a nuisance in some fields of application, such as micro-photography, interference and phase-contrast microscopy.
According to a feature of a preferred form of the invention, an aplanatical and achromatical image of the bright-field diaphragm can be obtained in the object plane of the microscope...
Aplanatical and achromatical imaging of the bright-field diaphragm 3 is due to the fact that the condenser according to the invention has two aspherical surfaces 7' and 7" and chromatically corrected cemented faces.
No mention of correction (or lack of correction) of the image of the aperture diaphragm at infinity and I still don't understand how aberrations of the field diaphragm image affect interference and phase-contrast but it at least answers the most basic question of which image is corrected.

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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#11 Post by patta » Mon Jul 26, 2021 11:29 am

Some guesses, slightly off from the previous interesting posts.
Summary, I think it should be corrected on aperture planes; opposite than the citation from previous post (?)
No links, just some hand bragging arguments:

'Achromatic' may mean that the illumination points at the condenser aperture are sent, on the object plane, in the same direction, for different colors. Or, the same, it is color corrected for aperture "at infinity".
This seems to hold at least for a Leitz 402a phase condenser I have here: with the Bertrand lens, I can't see any chromatic aberration from the annulus, or from the condenser iris borders.

On the other side, I can see some CA at the borders of the illuminated field, on the object plane. So it is not exactly achromatic in field planes.

"Aplanatic" I think is made the same; the condenser iris remains in focus (w Bertrand lens) both closed and open, and this means that the condenser has no aperture spherical aberration. With an Abbe non aplanatic, the iris instead would get out of focus at different apertures.

This aperture-aperture correction is the most logic construction I think, at least for phase contrast, where you need the condenser annulus projected correctly on the objective phase ring. But don't care much of its conjugate plane, so there may be left in some strong field aberrations.
It may be, that the condenser is designed to match/compensate the objective (the phase annulus is in the middle...) but anyway should still be an aperture-aperture pairing.

With Köhler, it is possible/easy to have one of the relay lenses, between lamp and condenser, compensating the field aberrations, such that the illumination can be made achromatic in all senses /planes. Just like objectives are not exactly achromatic, but the eyepiece kompensation fixes that.


Why this specification can't be found anywhere?

1- the condensers have less elements than objectives, but high NA, large field and long working distance; hard to get excellent aberration control; so "achromatic" may actually mean: we've patched the CA decently; enjoy it & don't be too picky on exactly where.


2- at high NA, moving up or down the condenser by half mm; or the slide, ~1mm thick of glass, can both send to hell an originally true achromatic/aplanatic light cone.

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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#12 Post by hans » Mon Jul 26, 2021 5:45 pm

patta wrote:
Mon Jul 26, 2021 11:29 am
"aplanatic" I think is made the same; the condenser iris remains in focus (w Bertrand lens) both closed and open, and this means that the condenser has no aperture spherical aberration. With an Abbe non aplanatic, the iris instead would get out of focus at different apertures.
My understanding of aplanatism is different, I think -- based only on low-order behavior (and derivatives with respect to field position) at the center of the field, so doesn't say anything about higher-order behavior like SA greater than third order or quadratic or faster variation in coma away from the center of the image or other aberrations?
patta wrote:
Mon Jul 26, 2021 11:29 am
This aperture-aperture correction is the most logic construction I think, at least for phase contrast, where you need the condenser annulus projected correctly on the objective phase ring. But don't care much of its conjugate plane, so there may be left in some strong field aberrations.
That makes the most sense to me also, but so far not backed up by anything I've read. What do you think of this theory:
hans wrote:
Wed Mar 31, 2021 2:30 am
Maybe the situation could be -- condenser and objective both optimized for imaging in the specimen plane, both image poorly FFP/BFP to infinity in between, but those aberrations tend to cancel well when considering combined imaging FFP to BFP?
---
patta wrote:
Mon Jul 26, 2021 11:29 am
With Köhler, it is possible/easy to have one of the relay lenses, between lamp and condenser, compensating the field aberrations, such that the illumination can be made achromatic in all senses /planes.
Good point, in the basic infinity-corrected design the field/collimator lens affects only the image of the field iris, further complicating things.

I have not had much luck yet getting non-confusing experimental results. My phase telescopes all have substantial aberrations of their own, and short focal lengths and barely-accessible focal planes of condensers plus corrections for slide thickness makes things difficult. I recently got a MILC (previously had DSLR so mirror was in the way) and wanted to try condenser directly as a camera lens to get a better idea of aberrations. Unfortunately it turns out ~4 mm thick filter/protective glass over the sensor is part of the micro four thirds spec and makes it impossible to use condensers at infinity focus.

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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#13 Post by apochronaut » Mon Jul 26, 2021 6:43 pm

compensating or kompensating = an inverse or negative correction designed into a downstream optic to compensate for a deliberate over correction built into an upstream optic.
correction = a positive correction designed into a downstream optic to overcome a deficiency of corrections in an upstream optic .
In the case of ca, which is the most common form of correction deficiency and the most common form of over correction ocurring due to more precise correction of other aberrations , the correcting optic will display blue interior ca and yellow exterior ca. With a compensating optic the colours reverse.

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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#14 Post by patta » Tue Jul 27, 2021 9:05 am

Hm, to image an hard-to-reach place, you can use a relay lens. I'm doing this a lot recently, to use microscope objectives in aperture space (like with a Bertrand Lens, that is basically a relay lens)
I've tried just now with the condenser, looking at its focal plane with a macro lens, to see if there are differences when used straight or inverted upside down (imaging the the aperture and field, respectively)
But no success, the system is misaligned and unstable (gaffer tape...) I tried an Abbe condenser (not-achromatic) and I can't even see clearly the chromatic aberration(s) that should be huge; and the images straight and inverted comes too different to be compared. See photo below for the setup.
Maybe keeping all mounted on the microscope stand is easier and better alignment.
I give up now and go to work.

I've read the CZJ condenser patent linked, it says quite explicitly that the design is achromatic-aplanatic corrected in field space. Which goes against my understanding.

Correction vs compensation, that may be possible to check, by looking at the color of the iris diaphragm in the images. That was my goal with the macro setup.
From my previous observations, the field illuminated by the Ach Apl condenser had a small but visible red/yellow exterior. To see it, I've heavily decentered the condenser and looked, on a frosted slide, at the illuminated field border with 10x objective. But I can't do the same trick to visualize the aperture border, hence the attempt with the macro lens.
The other observation, on aperture - I didn't see any chromatic aberration, looking at the (centered) condenser phase annulus with Bertrand lens - I think is less reliable, since there is the objective in the middle; and the Bertrand lens has very narrow aperture, so may reduce any aberration below visibility.

I'm still confused on what CA correction actually means. Because in intermediate optics, we have both position and direction of the rays - while when I pick up the image with a sensor, I care only about position (the ray must hit the right pixel; but can come from different directions).
For the spherical aberration, it is very messy because when you swap from field to aperture, those aberrations change name. Like, Spherical becomes Coma, or stuff like that. No clear ideas
.
Should do more reading on Relay optics.
I've read some material about Eyepieces, where is distinguished between field aberrations and "aberrations of the exit pupil", that is our aperture space. The famous "Nagler" telescope eyepiece had excellent field aberration control; but massive spherical aberration of the exit pupil, "Kidney bean" effect.
.
Coondenser focal plane.jpg
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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#15 Post by patta » Tue Jul 27, 2021 10:05 am

I think we are using "field aberrations" with two meanings:

1- aberrations in the field plane (opposed to aperture plane) which includes on-axis spherical
2 - aberrations in the field plane, away from the central axis; spherical aberration is excluded. Those are the official "field aberrations"

Lectures of Gross about aplanatism and pupil aberration - a bit complicated
https://www.iap.uni-jena.de/iapmedia/de ... 001550.pdf

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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#16 Post by apochronaut » Tue Jul 27, 2021 11:24 pm

patta wrote:
Tue Jul 27, 2021 9:05 am
The famous "Nagler" telescope eyepiece had excellent field aberration control; but massive spherical aberration of the exit pupil, "Kidney bean" effect.
I suspect Al Nagler didn't have Essential Tremor.

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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#17 Post by hans » Wed Jul 28, 2021 5:41 am

Not sure about precise classification of "field aberrations". I think I was only using two different meanings of "field":
  • identifying a particular set of conjugate planes in an approximately Köhler microscope
  • referring to field (as opposed to pupil) coordinates when aberrations are expressed in transverse ray form
I have mostly been looking at "Basic Wavefront Aberration Theory for Optical Metrology" by James C Wyant and Katherine Creath. The authors seem to have posted a few different versions different places, all with ugly OCR/reformatting, but the information is intact. The slides by Herbert Gross are nice for the many graphs and ray traces but I find it hard to follow through details without the accompanying lectures.
patta wrote:
Tue Jul 27, 2021 9:05 am
I've tried just now with the condenser, looking at its focal plane with a macro lens...
What macro lens, seems like that would require very close focus? I don't have any macro lenses but maybe the reversed prime plus telephoto would work for this. I tried that a long time ago with my Nikon DX lenses and should still have the adapter ring around somewhere...

Yeah, I also am not clear on relationship between field and pupil aberrations and what assumptions/approximations are necessary to deduce things about one from the other.

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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#18 Post by patta » Wed Jul 28, 2021 8:23 am

Hei! Nagler, maybe he's already part of US optical history... but he's also still alive, and working apparently. Maybe 90 years old? We can ask him to make some accessories for our microscopes. For example, a super-wide eyepiece with adjustable CA compensation, will surely be a bestseller (if under 1000$)

Gross lecture slides, yes, they're too short, my trick is to just look at the illustrations and skip the incomprehensible formulas.

The macro setup, it is a 150mm macro, will go to 1:1 magnification by itself; I've added some tube spacers to get to about 1.2:1, but just for convenience.
I won't recommend to go further in this direction; it takes time and effort; doesn't work well; this issue of the condenser is just a curiosity; we are just fiddling with optics, not manufacturing them; or, maybe...

A simpler test came out in the morning before coffee:
Project the image of condenser's aperture on the walls/ceiling, and look there for aberrations. Those will be aberration in "direction at the object plane", or pupil aberrations.
I've taken the microscope in my dark room (the bathroom); with the objective taken away, cranked up the illumination, and look at the projected light.
Results: the iris diaphragm isn't in focus! That means it isn't exactly in the aperture plane of the condenser.
The phase annulus, for the Achr condenser; the smaller annulus is quite out of focus as well. The largest annulus is in better focus (but not perfect yet); and shows a faint CA, this time with Blue on the exterior of the annulus.
interpretation of the results, we'll see. The fact that the large annulus is in focus, while the small no, I'd say that it is field curvature (the condenser is not "plan")

Other simple method:
Use the condenser as a loupe; look from its bottom to some test object (a finger, in my case) placed where the slide would be.
Results: the achromatic condenser shows no chromatic aberration; the simpler Abbe show chromatic aberration. wOOOw.

Third idea (I haven't tried, just guessing)
"Confocal": if you have Kohler, put a small pinhole in the field diaphragm
Focus at the image of this pinhole at the microscope, with large magnification.
A gross spherical aberration may be visible. Putting a darkfield stop at condenser's aperture, may make it more evident

The theory that both objective and condenser are built for imaging on the object plane, is sound, but sometimes we need also tight control of the illumination light cone directions; like with Phase or Darkfield. Truth may be in the middle...
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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#19 Post by patta » Wed Jul 28, 2021 9:24 am

BTW, a perfectly aplanatic and achromatic illumination, can be easily achieved as from "phone illumination" thread:
Remove the condenser, put the phone screen there instead, illuminating the slide directly. No optics -> no aberrations!
sponsoring my own threads, new low..

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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#20 Post by patta » Wed Jul 28, 2021 10:41 am

Last post, I promise...
(edit - no it won't)

I've done the last method, the "pinhole in object space"
I don't have a true Kohler illumination (with ö ), just put the lamp far away, covered with aluminium foil, with a small hole pierced by a literal pin.
The condenser focuses the image of the pinhole on the slide, no other Kohler optics.
Below the result of Leitz 0.90 Achr condenser, through the 40x objective; I've used a phase annulus in aperture & some defocus to get more dramatic effect. The actual pinhole is the faint blue dot in the middle. The picture is the whole field, phone camera at the eyepiece. If that is achromatic... :oops: :shock: :mrgreen:

Well, anyway, this pinhole turned out to be an excellent method to center precisely the condenser... by centering the CA!
.
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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#21 Post by apochronaut » Wed Jul 28, 2021 11:16 am

Using an objective, 20X is good, of known plan performance, bring the closed iris of the field diaphragm to focus while viewing through the eyepiece. Then open it. You can easily see how flat it's projected field is. Abbe not much. Aplanat much more.
Viewing front lens to back lens wide open at an evenly illuminatd window will show you the aperture.
Condensers can gain a lot by having a wider glass and therefore wider aperture. AO #1970 and PZO used this feature for better corrected wider fields.

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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#22 Post by hans » Wed Jul 28, 2021 4:52 pm

Projection with the condenser is a good idea, and seems like it should work in the other direction as well, projecting an image of a calibration slide for example. Will try tonight if I get a chance. Both of my bathrooms have windows.
patta wrote:
Wed Jul 28, 2021 8:23 am
The fact that the large annulus is in focus, while the small no, I'd say that it is field curvature (the condenser is not "plan")
apochronaut wrote:
Wed Jul 28, 2021 11:16 am
Using an objective, 20X is good, of known plan performance, bring the closed iris of the field diaphragm to focus while viewing through the eyepiece. Then open it. You can easily see how flat it's projected field is. Abbe not much. Aplanat much more.
I notice this curvature of the image of the field iris as well during normal use of the microscope. Not sure how much it relates to aplanatic/achromatic though, as far as I understand "aplanatic" and "plan" are entirely different things and aplanatic systems can still have field curvature. For example look at slide 10 in patta's link:
Conclusion: Aplanatism is only locally valid (axial and lateral)
Have not thought as much about achromatism yet, mostly aplanatism which seems like a more fundamental issue, relevant even to monochromatic light. But that is a quite a vibrant rainbow donut image you got...

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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#23 Post by apochronaut » Wed Jul 28, 2021 5:50 pm

Yes. Aplanat is free from spherical aberration and probably coma. Whether a condenser would benefit from flatness of field is difficult to know, since in Köhler the optimum condition at the diaphragm plane is defocus.

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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#24 Post by hans » Wed Jul 28, 2021 5:58 pm

Have you ever heard of a condenser advertised as plan? I don't remember coming across one yet.

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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#25 Post by apochronaut » Wed Jul 28, 2021 7:30 pm

No. This is an interesting one, though. I don't think plan but there are enough elements for it to be so. No claim of plan. It probably was being dovetailed to be used with the high magnification flat field apochromats George Aklin also patented but I don't think any of them ever went into production. You can see where Bausch & Lomb was headed though. The 125X flat field apochromat prototype is 1.4 N.A., so the condenser would be at least that. There must be a few around somewhere because the objectives exist.
https://patents.google.com/patent/US3743386A/en

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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#26 Post by hans » Wed Jul 28, 2021 10:32 pm

No explanation but there is a statement similar to the Zeiss patent:
As is well known in the art it is the function of a microscope condenser to form a sharp image of an illuminated field diaphragm in the specimen plane of the microscope objective.

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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#27 Post by patta » Fri Jul 30, 2021 7:46 am

Ok some more reasoning

- Spherical aberration (SA)
it needs to be corrected in the Object plane. At high NA, SA can get really nasty, so nasty that the condenser doesn't actually manage to project high NA light on the slide.
Here an article where it is explained with clear illustrations:
https://www.cambridge.org/core/journals ... B1F783D5AD
http://www.microscopy-uk.org.uk/mag/ind ... timal.html
With SA in the object plane, the condenser height needs be adjusted for each different objective and NA. With no SA, the condenser can stay in the same place.

Why SA is not important in the aperture plane: because in aperture, the microscope has small "aperture", so SA is small.
In the aperture planes, the "aperture" is the field. For an infinite microscope with tube length 200mm and field 20mm, the NA of aperture is constant for all objectives, about (20/2)/200 ~ NA 0.05. Compare it with NA 0.90 or 1.40 used in the object plane...
The "phase telescope/Bertrand lens" has smaller field baffle, so aberrations in aperture are rendered even smaller; mine here, has a pinhole of just 2mm diameter, leading to a 0.005 NA: no chances to check for SA; only distortion and lateral CA visible. And they may originate from the Bertrand lens itself, or the objective, who knows.

- Chromatic aberration (CA)
it needs to be corrected in both, aperture and field; but it is not so nasty apart for use in darkfield or COL.
In the object plane, lateral CA gives some color fringes at the border of the field; axial CA, something similar. The rainbow of post #20, is axial CA on the object plane.
In the aperture plane, CA gives heavy rainbows when used darkfield, and some color effects in non-monochromatic phase contrast.
That is I think the main reason for the existence of Cardioid condensers for darkfield - they are reflective optics, so no CA, nice darkfield at high NA.

- Telecentricity
That may be the main design feature of any condenser and it's never mentioned. The condenser must provide straight-up illumination cones for all the field (not only in the center!); it should not be a diverging beam (cones pointing outwards).
I think it is the issue that pops up when using low magnification objectives, with large field; the borders are darker, because the diverging light from the condenser isn't taken up by the objective.
Also, sometimes the image shifts (scale, enlarge) when focusing; non-telecentricity may be the culprit.
Phase contrast won't work away for the center, if condenser and objective don't have matching convergence. Darkfield too.
I've tried in the past different "condensers": microscope objectives; telescope eyepieces; and camera lenses. Objectives and eyepieces work ok (they're telecentric)
Camera lenses work bad, because they're not.
Telecentricity requires a front lens larger than the illuminated field; here's why large front lenses have an advantage.

- Why it is so difficult to find proper information about condenser aberration issues
Because of the "building on the shoulder of giants" mechanism
Most of the issues about condensers were sorted out in the 1800's. Today they're taken for granted and nobody remembers them. Only new improvements are reported, while the fundamentals are ignored. Internet is flooded with the basic, didactical, pre-digested description of the condenser: always the same and saying little.
A few optical designers must be aware of the deep complicated details - but the marketing people don't know. This "forgetting the basics" happens all the way in science and technology.

Some modern thorough discussion about condenser-mask-objective interplay can be found in Lithography literature.
The Americans, who seem to disregard the existence of any pre-WWII knowledge, had to re-discover the condenser design and sort out again its many issues. (no offence to any forum member...that's my personal experience with scientific papers; and other nationalities don't fare better) The resulting lithographic optics are impressive. Maybe it is good to forget & reinvent.

hans
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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#28 Post by hans » Fri Jul 30, 2021 5:42 pm

patta wrote:
Fri Jul 30, 2021 7:46 am
it needs to be corrected in the Object plane. At high NA, SA can get really nasty, so nasty that the condenser doesn't actually manage to project high NA light on the slide.
Makes sense, so basically SA of the uncorrected Abbe is so bad at high object plane NA that you don't really get anything resembling precise control of the illuminated field diameter relative to the small field.
patta wrote:
Fri Jul 30, 2021 7:46 am
In the aperture planes, the "aperture" is the field. For an infinite microscope with tube length 200mm and field 20mm, the NA of aperture is constant for all objectives, about (20/2)/200 ~ NA 0.05. Compare it with NA 0.90 or 1.40 used in the object plane...
On the image side, but on the illumination side the condenser images the aperture iris at higher NA when the illuminated field is large, I believe? For example MicroStar IV system goes up to 5 mm with ~10 mm EFL condenser at the normal height, so NA 0.25? (2.5 mm / 10 mm)

Regarding telecentricity, this is equivalent to saying that the limiting aperture stops are located at the condenser FFP and objective BFP as usual? Not sure it makes sense to say eyepieces themselves are inherently telecentric, rather projection of the intermediate image near infinity by the eyepiece is telecentric only because the objective BFP is the limiting aperture stop in the system? Actually I have read that newer camera lenses designed for digital (particularly mirrorless systems) are often telecentric on image side so maybe could work as condenser with modest NA?
patta wrote:
Fri Jul 30, 2021 7:46 am
Internet is flooded with the basic, didactical, pre-digested description of the condenser: always the same and saying little.
Yes, this seems to make good info harder to find.

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75RR
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Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#29 Post by 75RR » Fri Jul 30, 2021 6:28 pm

patta wrote:
Fri Jul 30, 2021 7:46 am
Internet is flooded with the basic, didactical, pre-digested description of the condenser: always the same and saying little.
Substitute condenser for just about anything you like and you have basically summed up the internet.

We google answers rather than read up on topics ... we equate convenience with depth ... that is our problem, not the internet.
patta wrote:
Fri Jul 30, 2021 7:46 am
A few optical designers must be aware of the deep complicated details - but the marketing people don't know.
Maybe we need to talk to these optical designers.
Zeiss Standard WL (somewhat fashion challenged) & Wild M8
Olympus E-P2 (Micro Four Thirds Camera)

hans
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Location: Southern California

Re: Condenser corrections aplanatic, achromatic, etc. -- which planes are they referring to?

#30 Post by hans » Fri Jul 30, 2021 6:47 pm

75RR wrote:
Fri Jul 30, 2021 6:28 pm
Maybe we need to talk to these optical designers.
There seem to be few books covering microscope optics at a level of detail that an optical designer working on even very basic microscopes would be familiar with. Of course reading general books about optics and understanding from first principles should be possible, but not easy. I see a lot of references to the books by Maksymilian Pluta but they are not easy to find. Has anyone looked at them?

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