Simple meniscus orientation question?

AgX · Nov 25, 2021

shutterfinger said:
Meniscus lens diagram from A Lens Collectors Vade Mecum; light travels the direction of the arrow.
View attachment 291812

This is NOT a meniscus lens.
(Neither a element or the whole lens forms a meniscus.)
What you show is a plain achromatic lens.

These are meniscus lenses:

EDIT:
at enlarging the image, Shutterfinger's sample seems to be a achromatic meniscus indeed, but still hardly any.

Here better examples of meniscus achromatic lenses

in the left picture the positive lense still is biconvex, in the right one already planoconvex. At a third version it would be a positive meniscus, thus forming an achromatic meniscus at its best, shapewise.

shutterfinger · Nov 25, 2021

My source A Lens Collectors Vade Mecum

Q15 is a typical Tessar. Different manufacturers that made their own have different curvature of the elements than the generic. Similar is likely for a Meniscus.

AgX · Nov 25, 2021

But sources may fail too... As I showed, your sample is hardly to be recognized as forming a meniscus.

shutterfinger · Nov 25, 2021

Take it up with them https://www.antiquecameras.net/lensvademecum.html
The info was/is close enough to get the OP on the correct assembly path of his lens.

reddesert · Nov 25, 2021

When people these days refer to achromats, they usually mean cemented doublet achromats, but when a lens catalog from 1895 calls something an "combination achromatic meniscus pattern," by achromatic they meant that two elements are used to color-correct the lens, without implying that it was cemented. The positive crown glass and negative flint glass elements are how nearly all achromats are constructed. The fact that the lens was air-spaced gives more reflective surfaces so more flare, but it also gave the lens designer a couple of extra degrees of freedom (or maybe allowed them to make it up from stock elements).

Figuring out which is the positive crown is easy, it's the one that makes an image by itself. The other one is negative. I have no experience with this lens and can't really tell much from the pictures, but I can make educated guesses:

- The lens when behind the shutter should have the negative lens in front and the positive lens behind (closer to the film). This reduces off-axis aberrations.

- The more strongly convex surface of the positive lens goes toward the film for the same reason. (In box camera lenses with a single meniscus element behind the shutter, the convex surface goes towards the film; if you reverse the lens the image gets blurrier in the corners.)

- The orientation of the negative lens in front is not as easily guessable, but now you only have two options to try. Use some film and try it both ways. Hopefully you'll get an in-focus image in the center either way, but one way will be sharper in the corners than the other.

AgX · Nov 25, 2021

If you have an achromat, cemented or not, that in its whole forms a meniscus, then the convex side should be to "the world" as nicely worded somewhere up.

AgX · Nov 25, 2021

shutterfinger said:
Take it up with them https://www.antiquecameras.net/lensvademecum.html
.

The optical characteristic of a meniscus lens, that makes it chosen as taking lens above a biconvex lens, is dependant on its meniscus curvature.

The point is that we understand what we see at some sources.

Reddesert made a good point in hinting that uncemented doublets can go through as achromats too. We typically have a cemented one at mind.

Nodda Duma · Nov 25, 2021

I was going to post pretty much exactly the same thing. With such a simple design there’s really only one good way to orient it.

reddesert said:
When people these days refer to achromats, they usually mean cemented doublet achromats, but when a lens catalog from 1895 calls something an "combination achromatic meniscus pattern," by achromatic they meant that two elements are used to color-correct the lens, without implying that it was cemented. The positive crown glass and negative flint glass elements are how nearly all achromats are constructed. The fact that the lens was air-spaced gives more reflective surfaces so more flare, but it also gave the lens designer a couple of extra degrees of freedom (or maybe allowed them to make it up from stock elements).

Figuring out which is the positive crown is easy, it's the one that makes an image by itself. The other one is negative. I have no experience with this lens and can't really tell much from the pictures, but I can make educated guesses:

- The lens when behind the shutter should have the negative lens in front and the positive lens behind (closer to the film). This reduces off-axis aberrations.

- The more strongly convex surface of the positive lens goes toward the film for the same reason. (In box camera lenses with a single meniscus element behind the shutter, the convex surface goes towards the film; if you reverse the lens the image gets blurrier in the corners.)

- The orientation of the negative lens in front is not as easily guessable, but now you only have two options to try. Use some film and try it both ways. Hopefully you'll get an in-focus image in the center either way, but one way will be sharper in the corners than the other.

Donald Qualls · Nov 25, 2021

I had thought there were three elements in two groups because as seen in the most recently posted image, the right side lens appears to be a doublet (apparently cemented joint visible on the ground edge), likely an achromat (else why bother with a doublet?).

I've rethought this a little, however; even if there's no cemented doublet, I'd go with @reddesert . I haven't seen an airspace achromat (other than possibly in a telescope eyepiece sold with a drug store telescope), but such would be easier to make than a cemented achromat meniscus, because no two surfaces have to match perfectly.

Spacing between the elements/groups is important, but the screw-in mount should take care of that -- one of those is slightly smaller diameter than the other, and they'll rest in the bottom and on a shoulder in the metal ring as I posted before.

There may be a need, after determining correct orientation, for some kind of cement to keep the elements from moving around in the ring; I'd use something reversible, if possible, like celluloid-base model glue ("Testor's Cement for Wood Models" or Ambroid, for instance, but easily removed with acetone or toluene).

Nodda Duma · Nov 25, 2021

Donald,

The Zeiss Planar T* 50 f/1.4 has air spaced achromats. They separated the flint and crowns of the traditionally cemented interior doublets to provide improved correction for opening the lens up to f/1.4.

gone · Nov 26, 2021

Keep good notes as you use the camera in different ways to figure out the proper lens orientations. One roll of testing will tell you how things go.

AgX · Nov 26, 2021

Donald Qualls said:
I haven't seen an airspace achromat (other than possibly in a telescope eyepiece sold with a drug store telescope), but such would be easier to make than a cemented achromat meniscus, because no two surfaces have to match perfectly.

Of the most common, house-brand 500mm tele-lens there is a version with a front achromat with huge airspace mounted in a sub-barrel. (The manafucturr though calls this assembly "2E 2G"...)

Simple meniscus orientation question?

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