PeteM wrote: ↑Thu Mar 14, 2024 7:22 pm
Here's a shot at summarizing the discussion of eyepiece corrections in a table. There are surely omissions and likely errors. Let me know and I'll update it.
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AO, Reichert, Leica eyepiece corrections.png
edit. I wrote all this before I saw your pictures but they confirm my suspicions.
I don't think the table represents the situation the way it comes out in my actual testing. There is also some information in various patents.
Firstly the F.N. of the various systems. F.N. is a confusing spec. It seems that in some references to it, the F.N. is objective dependent but in other references it is eyepiece dependent. Then there is the issue of the apparent field vs the actual field, not to mention the limitation of the ocular tube or the magnification of the eyepiece.
The B & L 15X U.W.F. eyepieces are a good case study. If you look at B & L literature related to their 15X U.W.F. eyepieces, the F.N. is listed as 20mm. They see the same field as the 20mm F.N.10X eyepieces do, yet becsuse they are 15X in order to achieve that, the image circle or apparent field is 1 1/2 times that of their companion 10X 20mm F.N. eyepieces. It serms then that to equalize the ground in terms of F.N., it should be the field stop of the 10X eyepieces that determines the F.N. , not the ocular tube size or apparent field , or theoretical objective image circle, and certainly if the objectives are being limited by the diameter of the optical tube.
So, in the case of each system you portray in your table I think it makes sense to establish an F.N. for the system plus a theoretical F.N. or objective image circle.
Secondly, the corrections. Corrections are very subtle because there are numerous aberrations that need to be corrected for and each one requires specific corrective attention that is potentially deleterious to another aberration . i think if any one of us saw the math associated with the evolution of a single eyepiece highly tuned to an
objective family, our jaws would resemble rhat of Eben Byers. When there is talk of correction, it almost always means correction for chromatic aberration. Nikon has carved a career out of trumpeting "chrome free" but did they ever say spherical free or curve free? No, because in order to correct completely in the objective for chroma, you have to undercorrect for spherical aberration and visa versa. Chroma correction gets one's attention because it is obvious. If you put an objective in a system that it isn't ca corrected for, anyone would notice that. If you put one in a system that it isn't sa corrected for, almost no one would notice that. They would go "wow these objectives aren't as sharp as the ones that came with the scope". Curvature of field? It is generally overlooked if slight but : in order to correct for ca in the objective , you would likely have to install other aberrations that would need to be corrected in the telan lens or a combination of the telan lens or the eyepiece. Leica is just being honest in stating the truth of optical design.
Your table needs some refinement.
The AO Cycloptic and Series 2/4 were not 100% harmonious in their eyepiece requirements and the series 10 needn't be in that section at all. The Cycloptic used a 147B 15X eyepiece, so there is something different about it's optics. For one thing it is infinity corrected. I would limit the section on the series 2/4 to just that,, the 2/4. It is relavent to the general pogression because it uses a corrective tube lens and has a tube length the same as the infinity systems, probably 7 3/16" originally, so just a bit over the 182mm mark. They didn't serm to be aiming for a f.n. target, probably doing as best they could with what they had. The objectives and eyepieces evolved in the late 1940's with the tube lens a further refinement that installed ca and curvature corrections, while extending the tube length without changing the magnification.. The eyepieces are close to neutral, maybe slightly corrective for ca and curvature. The field is about 19 .25 mm. A 34mm parfocal system.
The series 10/20 was the first professional infinity system and used 34mm parfocal objectives. They also introduced student lines as well. Originally, the objectives were close in design and planarity to those of the 2/4 but they seemed to know where they were headed because the #176 eyepiece was tuned to plan objectives and is very slightly compensating. It is about as compensating as the 146 is correcting but I would have to do a really careful and thorough assessment of all the AO Reichert eyepieces because they are all very close. The refinements are extremely subtle and lean into field curvature adjustments heavily, once the era of expected planarity arrived in the 60's.
Later in the decade, AO introduced the #176A eyepieces that were teamed with the standard achromats while the 176 went with the plan achromats.The ca corrections seem identical but there is an attempt to reduce the field curvature of the standard objectives with the 176A. .There was a limited release of infinity apochromats in the early 60's too and the 176A work well with those giving an unusually large 19mm field, which was a lot with apos in that era. The 176 are also 19mm.
In the early 70's Reichert Austria developed a 45mm parfocal infinity system which from my tests use the same corrections as the AO 34mm system. The Austrian system, since in the day Austria had about 1/2 the per capita GDP of the U.S. was created much more economically than it could have been done in the U.S. In fact Austria also introduced a 34mm parfocal system in the mid-70's. It was on the Neovar 2 platform, wheras the similar but a little more elaborate Diavar 2 used the 45mm objectives.The two systems might be roughly compared to the 100 and 400 series out of Buffalo. I don't know off hand what eyepieces were used in those but they were 23.2 mm tubes and likely WP 10/20. With the larger microscopes, the Univar and Polyvar using the same objectives , the eyepieces were WPK 10/24. I doubt if the K addition on those refers to compensation for ca. More likely it is teamed with plan as in plan compensating, due to the extra 2mm of peripheral field that the same objectives as used in the 20mm Diavar 2 had to cover.in the 30mm ocular design. There may have been a tweak of the curvature required. I can't really test that at the moment.
Anyway, despite the Uni and Poly microscopes having a 30mm eyepiece tube, they produced only a 24mm f.n. visual field with the 10X eyepieces. There may be field curvature limitations beyond that , even though the eyepiece tube would allow that.
In terms of corrections, when I install objectives from either the Univsr or the Polyvar in an AO 10 stand and use #176 eyepieces, they are perfectly corrected and flat, so my conclusion is that the corrections are the same. If I use Nikon CFW 18mm f.n. eyepieces, same result.
The AO 34mm plan objectives I have dug into by the way, Pete have an f.n. of 24mm, as per patents for them. The optical tube and eyepieces limit their plan performance. I suspect , that you could put an 11mm extension on a #1309 40X .66 advanced planachro for instance, thread it into a Polyvar nosepiece and it would be plan to the edge of the 24mm field and perform quite well. I don't know for sure but it would be interestingto try that.
The series 110/120 is just a wider field clone of the 10/20. The optical tube was widened and the # 180 eyepieces equally corrected to the levels of the #176, just with a 20mm f.n.
With the Series 400, things get odd and that was the main reason for the original post. While the eyepiece ca corrections are so close in the #181s used in the series 400 to the 176 or 180, they differ in curvature correction. Using a pair of 176 in a series 400 microscope results in field curvature. and the series 400 objectives are not fully compatible with the earlier infinity systems in terms of peripheral ca. Those objectives are also a 24mm image circle but possibly more fully corrected for curvature of field and or spherical aberration needing less in the telan lens but further ca correction in those corrector plates in the head. I am still assessing this. There has to be a reason for the change in engineering strategy and why Leica has continued so. This ca/sa/fc relationship in objective corrections is the axis of evil that all optical designers juggle. You correct for ca, then you have to deal with spherical and curvature down the line. Corrrect for curvature and you have to deal with ca and sa and so forth. This is the reason for ever increasing objective lengths. More glass choices make it easier but uf you look at some of the older designs, the AO 1029 100X planachro had lenses hanging out the back up into the nospiece and highly corrected even 45mm objectives are stuffed full, so the strategy to get as much correction in the objective results in bigger objectives.
Has anyone used a Nikon objective straight to the sensor at 200mm without a tube lens? I would be very surprised if they aren't correcting for curvature and or spherical aberration in the tube lens and they make heavy use of the embedded front lens design that they probably licensed from AO back in the day?