Lens for spherical focal plane

Sune Posselt · Oct 22, 2014

Hi Apug, this is my first post here, so I hope I'm posting in the right forum.

I was looking at pinhole panoramas, using a cylindrical film plane. Very cool stuff, but obviously suffering from severe edge distortion. So I was wondering what would happen if you had a hypothetical hemispherical film plane? Practical issues aside, wouldn't you have a perfect, distortion free, three dimensional representation of the scene?

That got me thinking, can you add a lens to increase light gathering? The pinhole I'm pretty sure would work, but when you add optics it gets over my head. What kind of lens (simple or compound) would work for this?

Nodda Duma · Oct 22, 2014

Hi I'm new to posting here too, but I'm an optical lens designer so I think I can answer your question.

For a truly hemispheric image plane, you'd want to use a monocentric lens. You can google DARPA monocentric lens and find the paper "Fiber-coupled monocentric lens imaging" to read about it.

But a hemispheric surface is a special case of a simple curved image surface. If you are allowed a curved image surface (and the optical designer is allowed to set the radius of curvature), then you can greatly simplify the optics.

The reason is that one of the fundamental aberrations that needs to be corrected is field curvature. Trying to image a hemispherical scene (that is, objects in focus) onto a flat image plane is not easy, and requires increasingly complex designs to provide imagery as sharp as that of on-axis.

If you could eliminate the need to correct field curvature by curving the focal plane onto the Petzval field, then you've eliminated one of the primary drivers behind the need to add elements to a camera lens design. A very simple design (like a landscape lens, ie Kodak Brownie camera) will give very satisfactory results over the entire image.

Also note: you would not eliminate distortion -- which is a change in magnification with field angle -- but it would become much easier to perfectly correct.

Sune Posselt · Oct 22, 2014

Thanks for the reply, that was exactly what I was looking for, though still a bit above my head.

It makes sense that lens design would be much easier without the need to correct for field curvature.

Do you know if there are any off the shelf lenses available that will let me experiment with this at a reasonable cost? It would be really cool to make an image on a concave surface, the larger and more curved the better. Even if I never get to mess with emulsion, it'd make a neat camera obscura experiment.

bdial · Oct 22, 2014

Some of the Kodak Brownies have a curved film plane. In particular, the "Bullet" brownies as seen here; http://www.brownie-camera.com/13.shtml do.
Not sure where you are, but I see them a lot in antique shops around the N.E. US.

Thomas Bertilsson · Oct 22, 2014

bdial said:
Some of the Kodak Brownies have a curved film plane. In particular, the "Bullet" brownies as seen here; http://www.brownie-camera.com/13.shtml do.
Not sure where you are, but I see them a lot in antique shops around the N.E. US.

So did the Agfa Klack 6x9 cameras, in case it would be of any help.

E. von Hoegh · Oct 22, 2014

Thomas Bertilsson said:
So did the Agfa Klack 6x9 cameras, in case it would be of any help.

And my Minox B...

Thomas Bertilsson · Oct 22, 2014

E. von Hoegh said:
And my Minox B...

Is that the little 8mm spy camera?

Nodda Duma · Oct 22, 2014

Yep a Kodak Brownie would be a good lens to experiment with. You get apparent and obvious performance improvement by curving the image plane.

Early brownies had flat planes. The Hawkeyes I think?? Or maybe just later Brownies had curved plane but same optics otherwise.

Sune Posselt · Oct 22, 2014

I was hoping for something with a much larger image circle, at least 8x10. And preferably with as much tighter curve.

Maybe it's possible to rig something up with scavenged elements - any ideas?

gzinsel · Oct 22, 2014

for pinholes: using a cyclinder container/ curved film plane. you can optimize pin hole for the max distance f /no. , or the closest distance. there lies in "the rub". No one size pinhole will optimize for the entire curved film plane. IMO, thats what make them cool. however, if you start adding objectives front or rear of the hole or both, other problems start coming into play. It would great to design your own lens, and grind down and polish your own glass. set in brass. !!! but I do not know how to do that. aside from how awesome I am at re-inventing the wheel, I am not much good here re-inventing lens a for curved film plane.. . . . But if you need help re-inventing the wheel- I can Help!!! cheers.

Gerald C Koch · Oct 22, 2014

Most Brownie box cameras used a simple meniscus (concavo-convex) lens. A few used a better concavo-convex cemented doublet. The lens was situated behind a simple Waterhouse stop and shutter with the concave side facing the subject. Many people who tinker with these cameras make the mistake of re-inserting the lens in the wrong direction. Simple lenses can be obtained from companies that sell science supplies.

Sirius Glass · Oct 22, 2014

No suggestions, but welcome to APUG anyway.

Nodda Duma · Nov 1, 2014

Here's an example of an 8x10 landscape lens prescription

With a crown glass (BK-7), a meniscus lens with the concave surface having a radius of curvature ~1 2/3 that of the convex surface will correct astigmatism. Placing an f/12 stop about 36mm behind this lens will correct absolutely for coma (the distance is important for coma...the stop diameter will reduce spherical and color). This is the classic Landscape Lens (Wollaston Landscape Lens) as found in the Kodak Brownie (scaled down of course).

If you use an achromat meniscus, then you correct spherochromatism and you have the Chevalier lens.

If you halve the power of the Chevalier and place a copy behind the stop so that the layout looks like a mirror image, then you correct lateral color and distortion as well. This gets you the Rapid Rectilinear or Aplanat (depending on who you talk to). It allows you to open up the stop to like an f/6 or f/8...hence "Rapid". That takes you to the late 1800s in photographic design.

Oh and of course you could always have a custom lens fabricated... But in single quantities it'd be expensive. In any type of quantity at all (like hundreds) they would be relatively inexpensive.

Hope this helps,
Jason

ic-racer · Nov 1, 2014

http://www.noblexcanada.com/noblexproducts.htm

Lens for spherical focal plane

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