Meniscus Lenses and Film Planes

[thuggins]

I just got an Agfa Clack, amazingly small and light for such big negatives. Apart from that the camera is known for its combination of meniscus lens and curved film plane. It is apparent that the convex surface of the lens faces out. The lens on the Baby Brownie is also convex out, and it also has a curved film plane.

Pictures on the interweb typically show the convex surface facing in, toward the film plane. I'm fairly certain that both the Brownie Hawkeye and Foldex 20 are configured that way (at least that's the way it went when I reassembled my Brownie Hawkeye). Both of those have a flat film plane.

I'm assuming that the convex out/curved film plane produces a better image, but its application is limited by restrictions on camera size and shape.

 

[ransel]

I think every Brownie box camera that I have removed the lens for cleaning, the concave side was facing out, toward the aperture and subject, kind of like a Wollaston Meniscus...



...but, I may be mistaken.

 

[Nodda Duma]

When you look at lens layouts, look at the position of the elements relative to the aperture stop, not the film plane. Look at how symmetrical the design is about the stop. Same or similar types of elements in front as in back inherently reduce distortion, coma, and lateral color. The interplay between stop location and element position are far more important to optical aberration correction and design performance than their position relative to the image plane.

Landscape lenses all have a single meniscus element with the concave surface facing the stop, whether the glass is in front or behind. Wollastons, Brownies, etc are all related in this way. The distance from the stop adjust astigmatism and corrects coma. The lens shape corrects spherical aberration. The stop diameter controls everything else. The curved film plane helps reduce tangential field curvature in a landscape lens but isn't necessary since tangential results in more pleasing slight circular swirly similar to (but not as pronounced as) a Petzval.

 

[chassis]

Both good explanations above.

 

[thuggins]

Landscape lenses all have a single meniscus element with the concave surface facing the stop, whether the glass is in front or behind. Wollastons, Brownies, etc are all related in this way. The distance from the stop adjust astigmatism and corrects coma. The lens shape corrects spherical aberration. The stop diameter controls everything else. The curved film plane helps reduce tangential field curvature in a landscape lens but isn't necessary since tangential results in more pleasing slight circular swirly similar to (but not as pronounced as) a Petzval.

That is an interesting point about the concave surface of the lens always facing the aperture, and we all know that aperture diameter has an impact on the image. But the comment about the curved film not being necessary doesn't make sense. Obviously, a curved film plane presents challenges in the design of the camera (e.g. no pressure plate and the overall body shape). If having the convex surface facing a flat film plane produced the same quality image, then all meniscus lens cameras would be made that way. But obviously they are not.

There must be a good reason that camera designers accepted the challenges inherent in a curved film plane. And there must be a reason that curved film planes seem to always be coupled with meniscus lenses that have the convex surface facing out

 

[Nodda Duma]

There's no real challenge to designing for a curved film plane. Just different stamping die for the stamped part.

The curved film isn't necessary for a good image, but I didn't say it doesn't help. And in fact, the comment you quoted includes my explanation of such.

 

[thuggins]

There's no real challenge to designing for a curved film plane. Just different stamping die for the stamped part.

The curved film isn't necessary for a good image, but I didn't say it doesn't help. And in fact, the comment you quoted includes my explanation of such.

But there are challenges, and I cited two of them. First, you can't have a pressure plate (or at least it would be very challenging to incorporate one). This is a big deal as pressure plates are normally considered important for the image and are a feature in even the most moderate quality cameras. As for the other challenges, I was not referring to the tooling, but the overall design. Designers have striven to make cameras "pocketable" for nearly a century. Having a bulging backside (on the camera, not the pocket) is not conducive to slipping it into a pocket.

While I appreciate your comments, they don't address the central question. The curved film plane appears to only be associated with one specific orientation of the meniscus lens. There must be a specific reason that these two features are associated with each other and there must be a reason that camera designers chose to go that route.

 

[Leigh B]

...pressure plates are normally considered important for the image and are a feature in even the most moderate quality cameras.

You're misinterpreting the function of the pressure plate.

It does not hold the film "flat". It controls the amount of film curvature.

All roll films curve, with the edges of the film being closer to the front of the camera.
Hasselblad, Rolleiflex, and Nikon all went to great lengths to manage that curvature.
I expect other manufacturers did likewise.

- Leigh

 

[Nodda Duma]

While I appreciate your comments, they don't address the central question. The curved film plane appears to only be associated with one specific orientation of the meniscus lens. There must be a specific reason that these two features are associated with each other and there must be a reason that camera designers chose to go that route.

My comments did address the central question. It's all about field curvature. As a lens designer myself, I can't really give you any better insight if you don't have a cursory knowledge of how aberrations work. Consider this an invitation and an opportunity to educate yourself: You can google search aberrations and field curvature in particular.. Once you've read up some, then what I said above may make more sense.

One more point: the lens element is usually found behind the stop because that provides for a shorter camera vs glass in front for a given focal length. That has nothing to do with a curved film plane. In fact, the aberrations are oh-so slightly better controlled with glass in front. Lens design is always a balancing act with other requirements (usually size, weight, and ease of manufacture). The curved film plane just improves field curvature correction by matching the film curvature to the Petzval field. If you open up a latter-day Six-20 Brownie or similar with curved film plane, you'll see your point about manufacturing complexity is moot. In fact modern cameras have a flat film plane, which should tell you the advantage is not significant.