Objectives with a Concave front element.

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apochronaut
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Objectives with a Concave front element.

#1 Post by apochronaut » Wed May 18, 2022 4:52 pm

I decided to start a new thread about this concave front lens design, with a more general focus than the one objective. Reading the original thread would make a good primer for this thread.viewtopic.php?f=5&t=15471
It seems that objectives with the concave front element design feature probably are mostly derived from the 1938 patent linked to in the above thread. That feature along with modifications to the rear lens, at least in the early days of the development of more planar objectives were seminal to that goal.
Although the patent was secured in 1938, both in memory and physical evidence, prior to 1960, I have been unable to identify any commercially available examples that I can positively say are beholding to that patent Possibly, it may have been too expensive to implement until later on or the goal of wide fields may have taken precedence. For sure by 1955, fields of 19-20 mm were possible from several companies albeit not plan. Commercially obtainable plan systems did not become an economic reality until later : I would say the mid 60's but I may be off a few years.
I would hope that since the patent went to Zeiss, they might have commercialized the concept early on but then again I believe German patents may have been extinguished for a period after the war and Zeiss obviously had a period of rebuilding.
I have assembled a group of objectives that were made between 1961 and the early 70's that all seem to have such a design. With some it is a little difficult to tell without breaking the objective down but they all have the concave front element. Although not all are fully plan, those that are not do seem to have superior planarity , some across a fairly wide field for their time. They might be called semi-plan today. Only 3 of the objevtives were made after 1970 but sll stem ftom designs that pre-date 1970.
One of the non plan objectives is a Zeiss Jena objective but I don't know it's age. Perhaps someone more Zeiss knowledgeable does. Another may be an older Zeiss design and is an oddity because it is a water immersion apochromat.Many of the Lomo short objectives were designs pilfered from East Germany along with equipment and people but I don't know if this is one of them. It just has the concave front element and isn't particularly plan in performance. It may be evidence of the use of the design at Zeiss beyond the patent. Someone might know.
I could not find any Leitz objectives with this feature although sold a 40X .95 apochromat 37mm parfocal objective a number of years ago that I am sure had a concave front element. It seemed to be a late 50's , maybe 60's objective?
Picture 1 is a group shot.
Picture 2 , the same group from above. Note the two far right objectives in the middle row. They are both very early AO cat. # 1023 planachros from 1966 or 7. The ring around the front lens is different on each, one being a paint and the other being a smoked dark glass.
Picture 3 is the 10X .30 apochromat, 45X . 80 apochromat and a 45X .66 achromat all from the very first production of the AO infinity scopes around 1961.
Picture 4 is of the Zeiss Jena 40X .95 no cover apochromat (mfg. date unknown) and the possibly Zeiss derived Lomo 70X 1.23 water immersion apochromat. The water immersion apochromat is the only one of the lot that does not show extra glass beyond the lens surface. A metal cone covers right up to the edge.
Picture 5 is the oldest Nikon objective with this feature, a grandaddy to the objective in the original thread. 40X .65 plan achromat from 1968 or thereabouts. This one also has a smoked dark glass quality to the front lens block, similar to the earlier AO objective above. When looking at it from an angle the glass looks to have a black non reflective surface but when looking straight at the lens there is a very low glow of light visible through it, although non when viewing through the back lens.
If anyone wants any particular details about any particular objective, just ask.
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Last edited by apochronaut on Thu May 19, 2022 7:19 am, edited 2 times in total.

Greg Howald
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Re: Objectives with a Concave front element.

#2 Post by Greg Howald » Thu May 19, 2022 2:36 am

I must say your collection is quite impressive. It's obvious that you have been gathering them for some time. Such designs were wonders in their time, and represent steps in the evolution of optics. I know little about these objectives but I want to thank you for sharing. Some one here with vastly more knowledge than I have is bound to chime in on this. But Bravo!

apochronaut
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Joined: Fri May 15, 2015 12:15 am

Re: Objectives with a Concave front element.

#3 Post by apochronaut » Thu May 19, 2022 7:23 am

It seems more that they actually collected me, Greg..... I reread my text above and realized it was unclear, so I edited it a bit.

MichaelG.
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Re: Objectives with a Concave front element.

#4 Post by MichaelG. » Thu May 19, 2022 8:44 am

Thanks for posting this, Apochronaut !

I would ask one question, if I may:
What is the simplest optical prescription that you have in that vast collection ?

The reason for asking goes back to my comment about the Nikon 40x on the other thread
… an objective which retails at a mere £138 is unlikely to be very sophisticated, methinks.

I am particularly interested to understand the optical design ‘from the bottom, up’

It may be that Nikon has optimised the mass-production of this front element, and can use it casually: But somehow that doesn’t ring true.

MichaelG.
Too many 'projects'

Adam Long
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Re: Objectives with a Concave front element.

#5 Post by Adam Long » Thu May 19, 2022 9:24 am

I have at least 4 in my Zeiss assortment.

25x /0.45 Plan Ph2
25x /0.65 Planapo
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40x F
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63x /1.25 Neofluar oil
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The last is the most obviously concave to the eye, and not the most user friendly design when it comes to applying and removing oil.

apochronaut
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Re: Objectives with a Concave front element.

#6 Post by apochronaut » Thu May 19, 2022 1:19 pm

MichaelG. wrote:
Thu May 19, 2022 8:44 am
Thanks for posting this, Apochronaut !

I would ask one question, if I may:
What is the simplest optical prescription that you have in that vast collection ?

The reason for asking goes back to my comment about the Nikon 40x on the other thread
… an objective which retails at a mere £138 is unlikely to be very sophisticated, methinks.

I am particularly interested to understand the optical design ‘from the bottom, up’

It may be that Nikon has optimised the mass-production of this front element, and can use it casually: But somehow that doesn’t ring true.

MichaelG.
The criteria I used for selecting those objectives was that they all had to be designed prior to 1970. Out of the bunch there is 1 achromat, 1micro plan (semi-plan) achromat, 1 flat feld achromat, 6 plan achromats, 2 apochromats, 1 apochromat with correction collar and 1 water immersion apochromat with correction collar. The Jena 40X .95 160/0 apochromat may be the oldest, or the Lomo W.I. apochromat but I don't have any references for that.

Pending further information about those two, at this point the AO 45X .66 cat.# 1078 achromat would be the simplest objective form and as old as any other of known manufacture date. It's design would be comparable to the 1990's Nikon 40X achro objective.

With the initial release of the catalogue for the AO infinity system ( 1961 or 2) it included 5 objectives only, arranged by cat. # 1075 through 1079. 4X, 10X, 20X, 45X and 100X, plus the two uncatalogued apochromats 10X .30 and 45X .80 As the system persisted for over 20 years, probably more than 50 objectives evolved eventually, with most of the 20 to 63X objectives displaying the concave front element.

This is what that early catalogue says about that # 1078 45X objective. It even has a photo of someone examining the front element with an inverted eyepiece and instructions on how to clean it, as well as a schematic. I don't have a scanner but when I can, I will upload a photo of the page.

" To achieve the high degree of flatness obtained with the new 45X infinity corrected objective, it was necessary to utilize a small concave front lens of short radius of curvature. The surface of this front lens can be readily cleaned as illustrated in figure 28 with a toothpick covered with cotton at the tip or with a Q tip....."

AO did not list any of their first issue of objectives as plan. The f.o.v. for the system was 19mm. By 1966 they issued a series of 5 plan objectives , # 1017, 1019, 1022, 1023 and 1024., 4X, 10X, 20X, 40X and 100X. Numbers 1022 and 1023, as illustrated in the pictures above ( 4 and two iterations of 1023 , 5 and 6 counting from the left in the middle group) had the concave front lens structure.

Just prior to that planachro release, they engraved plan achro on a couple of the first issue of objectives. The 1017 4X and 1019 10X can be found marked with nothing mostly or as plan achro.I have disassembled both the 1076 10X and 1019 10X and they are in fact the same objective in a different barrel, just barely plan to a 19mm field. Both have a flat front lens but around 1980 they released the cat. 1021( which interestingly they left a gap for in their numbering scheme way back in 1966), a fully plan to beyond 20mm and with a concave front element.

The early 45X achromat with the concave front element , despite being touted as having a high degree of flatness of field, I have never seen marked as plan and although it was maintained in the catalogues, always stayed as a simple achromat while 3 other 40X planachros were evolved plus a high N.A. 40X advanced achromat and a 40X planapo.

The front lens form is reminiscent of the top lens of a condenser with the rear lens surface about 3 times that of the front and with instead of a flat top lens, a concave one. It is a thick meniscus lens, followed by a crown/flint, convex/plano achromatic doublet about 3 times the diameter of the entire front lens, or 6 times the diameter of the front lens surface.
A basic coated achromat yet with maybe better than average planarity than it's contemporaries.

tpruuden
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Re: Objectives with a Concave front element.

#7 Post by tpruuden » Tue May 24, 2022 11:15 am

How about concave front element condenser?
Relatively modern Leica:
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apochronaut
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Re: Objectives with a Concave front element.

#8 Post by apochronaut » Tue May 24, 2022 11:25 am

I have seen that with low N.A. condensers before. I would surmise that it is not abbe and has a flatter field, even if it is a 2 lens . Is it a dry condenser?

tpruuden
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Re: Objectives with a Concave front element.

#9 Post by tpruuden » Tue May 24, 2022 5:21 pm

Multi-lens condenser, this is the top part and at least 3..4 lenses total for the condenser.

apochronaut
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Re: Objectives with a Concave front element.

#10 Post by apochronaut » Tue May 24, 2022 5:45 pm

When you say top part do you mean it has a lens that is left in the condenser body when the threaded lens pack has been removed?

tpruuden
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Re: Objectives with a Concave front element.

#11 Post by tpruuden » Tue May 24, 2022 8:45 pm

At least one lens in the condenser body, not sure if it is cemented doublet or something else.

apochronaut
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Re: Objectives with a Concave front element.

#12 Post by apochronaut » Wed May 25, 2022 12:12 am

The most common design for an achromat I have seen does use a double convex doublet at the front, followed by a reversed meniscus, then a convex plano, in order of the lens surfaces from front to back ; essentially the reverse of a simple achromat objective. The concave back surface of that condenser probably means that the front curvature of that element is less extreme than the convex surface of a flat top achromat condenser. The middle element or elements less curvature too and potentially more field flattening effect. That should be a very nice flatter field achromat condenser.

tpruuden
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Re: Objectives with a Concave front element.

#13 Post by tpruuden » Thu May 26, 2022 10:02 am

From Leica DMR broshure, it may be the the condenser optical scheme, the massive front element seems quite similar:
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apochronaut
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Re: Objectives with a Concave front element.

#14 Post by apochronaut » Thu May 26, 2022 12:44 pm

Thanks for digging that up. Quite interesting but somewhat predictable. Just as with a concave faced objective the condenser has a similar form. Normally, with a high N.A. dry achromat aplanat there are two lenses up top and a doublet below.
The top lens/front lens in all in examples of these I have either broken down or reviewed the schematics of has a typical conical profile of a thick meniscus lens, replacing a plano convex/meniscus composite.

apochronaut
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Re: Objectives with a Concave front element.

#15 Post by apochronaut » Thu May 26, 2022 2:55 pm

This a schematic from the 1961 or 62 AO series 10 Microstar catalogue.
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