Reichert 181 eyepiece field curvature

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hans
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Reichert 181 eyepiece field curvature

#1 Post by hans » Sun Aug 02, 2020 8:29 am

I have seen mentioned several places that the 181 eyepiece may have some field flattening effect, in particular posts #10 and #12 by apochronaut and wabutter in this thread:
viewtopic.php?f=5&t=7457

I modified an eyepiece to allow easy access to the intermediate image plane for experimentation. The bottom surface is now flush with the factory-machined field stop:
reichert-181-mod.jpg
reichert-181-mod.jpg (87.61 KiB) Viewed 2392 times
Test subject is a 1 mm grid laser-printed on ordinary paper with LED backlight:
reichert-181-fc-setup.jpg
reichert-181-fc-setup.jpg (130.24 KiB) Viewed 2392 times
The field curvature and pincushion distortion are noticeably worse than what is seen when the eyepiece is working together with the rest of the optics in the 410:
reichert-181-fc.jpg
reichert-181-fc.jpg (175.52 KiB) Viewed 2392 times
From this it seems pretty clear that camera attachment must involve a 181 or some other element with similar field curvature to get optimal results.

MichaelG.
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Re: Reichert 181 eyepiece field curvature

#2 Post by MichaelG. » Sun Aug 02, 2020 10:13 am

Excellent forensic investigation, Hans
... Thanks for sharing the result

Impecunious amateurs need to know these unpublished details !!

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Element 56
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Re: Reichert 181 eyepiece field curvature

#3 Post by Element 56 » Sun Aug 02, 2020 4:37 pm

Hi Hans,
Thank you for sharing this.
I noticed that my Cycloptic stereo scopes have pretty bad field curvature. Funny I used them for years for polishing and even some engraving but didn't notice it until I was working on a collimation issue on one of them one day. I haven't really looked into the issue much since I've pretty much phased those scopes out but I would like to try some different eyepieces some time to see if it improves.
Kirby

apochronaut
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Re: Reichert 181 eyepiece field curvature

#4 Post by apochronaut » Sun Aug 02, 2020 4:46 pm

MichaelG. wrote:
Sun Aug 02, 2020 10:13 am
Excellent forensic investigation, Hans
... Thanks for sharing the result

Impecunious amateurs need to know these unpublished details !!

MichaelG.
Habitually poor amateurs? Which such amateurs are you thinking about?

Scarodactyl
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Re: Reichert 181 eyepiece field curvature

#5 Post by Scarodactyl » Sun Aug 02, 2020 5:03 pm

Element 56 wrote:
Sun Aug 02, 2020 4:37 pm
Hi Hans,
Thank you for sharing this.
I noticed that my Cycloptic stereo scopes have pretty bad field curvature. Funny I used them for years for polishing and even some engraving but didn't notice it until I was working on a collimation issue on one of them one day. I haven't really looked into the issue much since I've pretty much phased those scopes out but I would like to try some different eyepieces some time to see if it improves.
Kirby
15x uwfs work nicely. If you use them with non-compensating optics (like a Wild makrozoom lens) you see very pronounced curvature so you know a lot of compensation is happening there.

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Re: Reichert 181 eyepiece field curvature

#6 Post by apochronaut » Sun Aug 02, 2020 5:33 pm

Compensation for ca in an eyepiece sometimes exists independantly from correction associated with curvature of field. Many eyepieces exist that are designed to receive a well corrected internediate image and thus are not required to compensate for any residual ca. However, a bunch of such eyepieces can all have differing levels of curvature of field from barrel through neutral to pincushion.

They may work well in a non native application from the standpoint of freedom from off axis ca but impose too much additive curvature or not enough cancellation if curvature to be truly useful as a flat field eyepiece.

MichaelG.
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Re: Reichert 181 eyepiece field curvature

#7 Post by MichaelG. » Sun Aug 02, 2020 5:43 pm

apochronaut wrote:
Sun Aug 02, 2020 4:46 pm
Habitually poor amateurs? Which such amateurs are you thinking about?
No ... amateurs “having little or no money” [O.E.D. definition]
Perhaps “impecunious” means something different your side of the pond.

MichaelG.
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hans
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Re: Reichert 181 eyepiece field curvature

#8 Post by hans » Sun Aug 02, 2020 5:57 pm

apochronaut wrote:
Sun Aug 02, 2020 5:33 pm
Compensation for ca in an eyepiece sometimes exists independantly from correction associated with curvature of field. Many eyepieces exist that are designed to receive a well corrected internediate image and thus are not required to compensate for any residual ca.
That does seem to be the case in the 410/181 combination, since the CA I can see in the eyepiece alone is much less than the difference in CA I see with/without that corrector element thing at the base of the eyepiece tube. One possible issue with this test though, if the smartphone camera's EXIF data is to be believed, the aperture is 2.5 mm (4.5 mm focal length, f/1.8 ratio), somewhat larger than would be passing through the eyepiece in the 410 where the exit pupil diameter is limited to more like 1-2 mm by the objective N.A., I think?
apochronaut wrote:
Sun Aug 02, 2020 5:33 pm
However, a bunch of such eyepieces can all have differing levels of curvature of field from barrel through neutral to pincushion.
Are the curvature of focal plane and barrel vs. pincushion distortion essentially the same effect optically, at least for relatively simple lens combinations? I hadn't thought about it that way but it makes sense, I suppose.

hans
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Re: Reichert 181 eyepiece field curvature

#9 Post by hans » Sun Aug 02, 2020 6:05 pm

MichaelG. wrote:
Sun Aug 02, 2020 5:43 pm
No ... amateurs “having little or no money” [O.E.D. definition]
Or more realistically, "amateurs who like to think they are saving money while continuing to purchasing more broken microscopes, tools, calibration slides, LEDs, phase telescopes..."

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Re: Reichert 181 eyepiece field curvature

#10 Post by apochronaut » Sun Aug 02, 2020 6:17 pm

Since both barrel and pincushion distortion can be created due to an aperture restriction, one fore and one aft of a lens system, as fields got wider while the tube diameter didn't, eyepieces would have had to be tailored to specific levels of peripheral distortion as a corrective device.

Even with the CF, CFN and other objectives with that design initiative, although the claim is that they are ca free, they still are specifically tuned to an eyepiece design. Would that be because there is some curvature in the system that needs a specific degree of correction in the eyepiece, maybe?

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Re: Reichert 181 eyepiece field curvature

#11 Post by apochronaut » Sun Aug 02, 2020 6:18 pm

hans wrote:
Sun Aug 02, 2020 6:05 pm
MichaelG. wrote:
Sun Aug 02, 2020 5:43 pm
No ... amateurs “having little or no money” [O.E.D. definition]
Or more realistically, "amateurs who like to think they are saving money while continuing to purchasing more broken microscopes, tools, calibration slides, LEDs, phase telescopes..."
Why does sawing an eyepiece in half help them out?

hans
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Re: Reichert 181 eyepiece field curvature

#12 Post by hans » Sun Aug 02, 2020 6:47 pm

apochronaut wrote:
Sun Aug 02, 2020 6:17 pm
Even with the CF, CFN and other objectives with that design initiative, although the claim is that they are ca free, they still are specifically tuned to an eyepiece design. Would that be because there is some curvature in the system that needs a specific degree of correction in the eyepiece, maybe?
The issue of where in the overall system is it most economical (or even possible, in some cases?) to correct specific aberrations is interesting. It would be nice to find a not-too-technical reference that outlined common heuristics used by optical designers to decide.
apochronaut wrote:
Sun Aug 02, 2020 6:18 pm
Why does sawing an eyepiece in half help them out?
To save them the necessary expense of sawing one of their own eyepieces in half out of curiosity? Also, I'm going to object to the crudeness implied by the term sawing. I used shears, cutting carefully in a spiral, followed by a file.

But joking aside, the eyepiece actually still works fine since the seating depth is controlled by a shoulder on the outer diameter, not the part I cut off. There is a little more angular play which could probably be "fixed" with a layer of tape.

Zuul
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Re: Reichert 181 eyepiece field curvature

#13 Post by Zuul » Sun Aug 02, 2020 7:19 pm

The following image was shot through the binocular eyepiece of my 410 using my iPhone. To my eye, it shows similar distortion to your eyepiece test despite having the benefit of the entire system of corrections. Is the curvature actually an interaction between the eyepiece and the phone?

Image

hans
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Re: Reichert 181 eyepiece field curvature

#14 Post by hans » Sun Aug 02, 2020 8:26 pm

Zuul wrote:
Sun Aug 02, 2020 7:19 pm
The following image was shot through the binocular eyepiece of my 410 using my iPhone. To my eye, it shows similar distortion to your eyepiece test despite having the benefit of the entire system of corrections. Is the curvature actually an interaction between the eyepiece and the phone?
Is that the 40X plan achro? Mine has some residual pincushion like that also (less than the eyepiece alone, I think, but a more careful comparison would be a good idea) while the 4X and 10X come out much straighter. I have not spent nearly as much time looking through the 100X, just got some immersion oil a couple days ago.

MichaelG.
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Re: Reichert 181 eyepiece field curvature

#15 Post by MichaelG. » Sun Aug 02, 2020 8:41 pm

apochronaut wrote:
Sun Aug 02, 2020 6:18 pm
hans wrote:
Sun Aug 02, 2020 6:05 pm
MichaelG. wrote:
Sun Aug 02, 2020 5:43 pm
No ... amateurs “having little or no money” [O.E.D. definition]
Or more realistically, "amateurs who like to think they are saving money while continuing to purchasing more broken microscopes, tools, calibration slides, LEDs, phase telescopes..."
Why does sawing an eyepiece in half help them out?
.

Sorry but I think this is getting unnecessary

I complimented, and thanked, Hans for taking the initiative to actually investigate something, and demonstrate clearly the nature of the ‘correction’ in that eyepiece.
I have never seen this documented ... so I am wiser today than I was yesterday.
[ if you have the Reichert specification available, I would be delighted to see it ]

As for the impecunious aspect: Institutional purchasers and wealthy amateurs can simply buy what the manufacturer recommends ... I, for one, am not in that league; so it is interesting and useful for me to understand how things have been achieved.

The recent discussion about using Nikon CF objectives in ‘macro lens’ mode, rather than throwing money at a fully matched system is a case in point.

MichaelG.
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MichaelG.
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Re: Reichert 181 eyepiece field curvature

#16 Post by MichaelG. » Sun Aug 02, 2020 8:56 pm

apochronaut wrote:
Sun Aug 02, 2020 6:17 pm
Even with the CF […] they still are specifically tuned to an eyepiece design. Would that be because there is some curvature in the system that needs a specific degree of correction in the eyepiece, maybe?
Glad you mentioned that ... because I was disappointed to see no response from you when I asked for details [in Louise’s discussion].

Does your hanging question “maybe?” demonstrate that it is undocumented ?
... if so, that confirms the merit of practical investigation.

MichaelG.
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hans
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Re: Reichert 181 eyepiece field curvature

#17 Post by hans » Sun Aug 02, 2020 9:19 pm

MichaelG. wrote:
Sun Aug 02, 2020 8:56 pm
apochronaut wrote:
Sun Aug 02, 2020 6:17 pm
Even with the CF […] they still are specifically tuned to an eyepiece design. Would that be because there is some curvature in the system that needs a specific degree of correction in the eyepiece, maybe?
Glad you mentioned that ... because I was disappointed to see no response from you when I asked for details [in Louise’s discussion].
If people using them outside the intended microscope system are generally also doing focus stacking, and field curvature is the only aberration corrected in the eyepiece, then maybe it easily goes unnoticed and/or simply doesn't matter?

Hobbyst46
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Re: Reichert 181 eyepiece field curvature

#18 Post by Hobbyst46 » Sun Aug 02, 2020 9:32 pm

I find these discussions of which eyepiece for which objective of practical interest.
I hope and trust (in the words of the great D. Runyon) that LouiseScot gets her CF Nikon objective, enjoys an improvement of the photo setup and let us know the results.
Zeiss Standard GFL+Canon EOS-M10, Olympus VMZ stereo

MichaelG.
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Re: Reichert 181 eyepiece field curvature

#19 Post by MichaelG. » Sun Aug 02, 2020 9:41 pm

hans wrote:
Sun Aug 02, 2020 9:19 pm
If people using them outside the intended microscope system are generally also doing focus stacking, and field curvature is the only aberration corrected in the eyepiece, then maybe it easily goes unnoticed and/or simply doesn't matter?
My example was a single shot, with improvised tubes, and I would certainly say that the result is “good enough for Jazz”

MichaelG.
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apochronaut
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Re: Reichert 181 eyepiece field curvature

#20 Post by apochronaut » Sun Aug 02, 2020 11:02 pm

MichaelG. wrote:
Sun Aug 02, 2020 8:56 pm
apochronaut wrote:
Sun Aug 02, 2020 6:17 pm
Even with the CF […] they still are specifically tuned to an eyepiece design. Would that be because there is some curvature in the system that needs a specific degree of correction in the eyepiece, maybe?
Glad you mentioned that ... because I was disappointed to see no response from you when I asked for details [in Louise’s discussion]
Does your hanging question “maybe?” demonstrate that it is undocumented ?
... if so, that confirms the merit of practical investigation.

MichaelG.
Nikon doesn't admit to that in any literature and I don't have the parts handy but I did note that they also make a CWD eyepiece that they vaguely recommrnd for low power CF planapo objectives. The translation is murky but it seems that they are different somehow. If there are is no ca compensation required, then the CFD must be correcting for something else differently, than the CFW. Field curvature? I was hoping Viktor could answer that question better.

I have a bunch of eyepieces here that were intended for infinity corrected systems. Systems that are highly corrected by the time the eyepiece gets involved. With few exceptions, they all have subtle differences, mostly to do with the degree of effect on field flatness, when they are moved to a system they were not originally designed for. Sometimes the difference is subtle and tolerable, the periphery bending slightly backwards for instance but sometimes it is extreme. I would wager that the CWF eyepieces are , like those , critically adjusted for field flatness to correct for the CF objective's tendencies.

Zuul
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Re: Reichert 181 eyepiece field curvature

#21 Post by Zuul » Sun Aug 02, 2020 11:07 pm

hans wrote:
Sun Aug 02, 2020 8:26 pm
Is that the 40X plan achro? Mine has some residual pincushion like that also (less than the eyepiece alone, I think, but a more careful comparison would be a good idea) while the 4X and 10X come out much straighter. I have not spent nearly as much time looking through the 100X, just got some immersion oil a couple days ago.
It is the 40x PlanAchro, and so is this next shot. It was taken without the CA corrective optic, obviously. Do you see pincushion in it?

Image

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Re: Reichert 181 eyepiece field curvature

#22 Post by hans » Sun Aug 02, 2020 11:52 pm

Zuul wrote:
Sun Aug 02, 2020 11:07 pm
It is the 40x PlanAchro, and so is this next shot. It was taken without the CA corrective optic, obviously. Do you see pincushion in it?
It looks like there may be a bit less, but also the image is cropped showing less of the field than the first? I guess it would not be surprising if that corrective plate had some effect on field curvature also.

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Re: Reichert 181 eyepiece field curvature

#23 Post by Zuul » Mon Aug 03, 2020 12:07 am

hans wrote:
Sun Aug 02, 2020 11:52 pm
It looks like there may be a bit less, but also the image is cropped showing less of the field than the first? I guess it would not be surprising if that corrective plate had some effect on field curvature also.
That shot was done direct projection. No eyepiece at all. I don’t have one with the 40x that isn’t cropped, but here is the Reichert 25x both ways.
(The 40x is cropped because I used a teleconverter to fill the frame. It’s not a tight crop. The corners are close to the field stop.)

Image
Reichert 25x direct projection to APC

Image
Reichert 25x with 1.4x TC to APC

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Re: Reichert 181 eyepiece field curvature

#24 Post by Zuul » Mon Aug 03, 2020 12:13 am

I just rebuilt the 410’s focus mechanism so I’m behind on experimenting with the CA de-aberrator. It is a real shame that you can’t take that lens out non-destructively. I would prefer to incorporate it into my photo tube as-is, but that’s not a trivial endeavor.

If I could eliminate the CA and get the flatness I see in those shots, that would satisfy me. I’m getting bored with photos of micrometers, and I don’t plan on shooting microscopic brick walls. :D

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Re: Reichert 181 eyepiece field curvature

#25 Post by hans » Mon Aug 03, 2020 2:05 am

Zuul, I don't think I have enough theoretical understanding to deduce much about the eyepiece from the configurations with many factors changed. Just a few rambling thoughts:
  • The question apochronaut brought up of, how closely are geometric distortion and field curvature coupled? Makes sense they would be closely related in simple cases, but I think there are counterexamples in more complicated designs, for example camera lenses which achieve very good planarity on the sensor side when focused at infinity but have severe geometric distortion. No idea how relevant that is to microscopes.
  • In configurations where the severe transverse CA is present it seems a bit difficult to distinguish that from field curvature.
  • It does look like the corrective element behind the eyepieces may have some effect on geometric distortion, meaning the distortion coming out of the thick glass element (prism/splitter in the binocular case, or glass cylinder in the camera port) is maybe not as bad as implied by the test of the eyepiece alone. But again, the transverse CA in that configuration makes it hard to tell what else is going on.
  • I think all of this strongly favors an afocal setup with the whole eye tube and corrective element borrowed from another head for use on the camera port.
  • Direct projection with corrective element and 181 -- I am skeptical whether the working distance can be changed enough for reasonable projection to APS-C, for example, without disturbing the corrections. However, the corrective element does give a whole other degree of freedom vs. typical eyepiece projection, so maybe good results are possible. (What I mean by extra degree of freedom, is, for example, do you move the corrective element and eyepiece together to achieve focus? Or just the eyepiece? Or some combination?)
  • I don't think I am going to experiment further with direct projection using the 181 unless afocal using the Nikon DX 35mm 1:1.8G I already own doesn't work well for some reason.

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Re: Reichert 181 eyepiece field curvature

#26 Post by Scarodactyl » Mon Aug 03, 2020 3:40 am

apochronaut wrote:
Sun Aug 02, 2020 11:02 pm
Nikon doesn't admit to that in any literature and I don't have the parts handy but I did note that they also make a CWD eyepiece that they vaguely recommrnd for low power CF planapo objectives. The translation is murky but it seems that they are different somehow. If there are is no ca compensation required, then the CFD must be correcting for something else differently, than the CFW. Field curvature? I was hoping Viktor could answer that question better.
This was recently explained to me on facebook--per that, it is more a matter of CFW eyepieces introducing some slight CA of their own, while the Ds were more fully corrected. They didn't sell many because they were expensive (and presumably not really in demand anyway, since the eye doesn't care as much about minor stuff). It would probably be pretty important if attempting afocal with a viewing eyepiece.

MichaelG.
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Re: Reichert 181 eyepiece field curvature

#27 Post by MichaelG. » Mon Aug 03, 2020 6:34 am

apochronaut wrote:
Sun Aug 02, 2020 11:02 pm

Nikon doesn't admit to that in any literature and I don't have the parts handy […]
Sometimes the difference is subtle and tolerable, the periphery bending slightly backwards for instance but sometimes it is extreme. I would wager that the CWF eyepieces are , like those , critically adjusted for field flatness to correct for the CF objective's tendencies.
.
Thanks for that ^^^
I think we are on the same page now.

The reason that my ‘macro lens’ test with the 20x WI objective lens worked ‘good enough’ might be that I was only capturing about a third of the primary image diameter ... making Petzval curvature trivial.

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Re: Reichert 181 eyepiece field curvature

#28 Post by MichaelG. » Mon Aug 03, 2020 6:59 am

apochronaut wrote:
Sun Aug 02, 2020 11:02 pm
... but I did note that they also make a CWD eyepiece that they vaguely recommrnd for low power CF planapo objectives. The translation is murky but it seems that they are different somehow. If there are is no ca compensation required, then the CFD must be correcting for something else differently, than the CFW. Field curvature? I was hoping Viktor could answer that question better.
.
For what it’s worth ... This, from the LABOPHOT manual, is the best information I have found to date:
.
Nikon CF series Eyepieces
Nikon CF series Eyepieces
F10F1575-F3B5-4323-B2C8-3FC2C5AAC6F0.jpeg (151.49 KiB) Viewed 2197 times
.
MichaelG.
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Re: Reichert 181 eyepiece field curvature

#29 Post by MichaelG. » Mon Aug 03, 2020 7:46 am

Zuul wrote:
Mon Aug 03, 2020 12:13 am
... and I don’t plan on shooting microscopic brick walls. :D
:oops:
MichaelG. :geek:
.
Ref. viewtopic.php?f=5&t=7871&p=69405&hilit= ... all#p69405
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Re: Reichert 181 eyepiece field curvature

#30 Post by apochronaut » Mon Aug 03, 2020 11:52 am

MichaelG. wrote:
Mon Aug 03, 2020 6:59 am
apochronaut wrote:
Sun Aug 02, 2020 11:02 pm
... but I did note that they also make a CWD eyepiece that they vaguely recommrnd for low power CF planapo objectives. The translation is murky but it seems that they are different somehow. If there are is no ca compensation required, then the CFD must be correcting for something else differently, than the CFW. Field curvature? I was hoping Viktor could answer that question better.
.
For what it’s worth ... This, from the LABOPHOT manual, is the best information I have found to date:
.
F10F1575-F3B5-4323-B2C8-3FC2C5AAC6F0.jpeg Michael G
That's why I said murky. If CWD have prominent image flatness then some other prominence ( CFW) must be less flat. It's like Everest and Annapurna shrouded in cloud. There is an implication that CWD eyepieces provide better ca correction for the CF planapo objectives , which begs the question : if the objectives are supposed to be chroma free, then why do they need correction for chromatic aberration? When it comes to the CFW eyepieces , the information is very revealing. It seems the fact that they come with rubber eyeguards is their most important feature.
Last edited by apochronaut on Mon Aug 03, 2020 12:05 pm, edited 2 times in total.

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