Focusing on curved film plane

[Jeff Searust]

I am building a 6x9 camera with a curved film plane. The lens I am using is out of an old agfa. The basic design of the camera is based on that of the Agfa Clack or Chief where the film plane is curved a bit.

Does anyone have info on the formula to use to figure the curve of the film plane? Or for that matter I am now up against the problem of how I define where the film plane is to place the lens and have it focus correctly... any ideas? Can I just use a flat ground glass at the center and figure out the focus there or do I need to figure out how to bend a piece of ground glass or plastic to make sure it's focusing properly across the length of the plane? :confused:

 

[keithwms]

use a pinhole

 

[Jeff Searust]

Already got an f128 pinhole Isolette. I want to try something more pedestrian, and I need to make a camera out of the lens I took off the Isolette.

 

[Steve Smith]

I have always thought that the radius of the curve is the same as the focal length of the lens.

I have no fomulae or reference to back this up, it is just what I think would be logical.

However, if you are using the Isolette lens I think you will need a flat film plane. All of the Isolettes I have owned have had flat film planes.



Steve.

 

[Nicholas Lindan]

the formula to use to figure the curve of the film plane?

The film plane is curved to correct, somewhat, the spherical aberration typical of the single element meniscus and plano-convex lenses used in box cameras. Ideally the film plane would be spherical. Some astronomical camera/telescopes do indeed use spherical film planes.

If the lens is corrected for spherical aberration then the film plane should be flat. The lens from an Agfa Isolette is corrected for spherical aberration and curving the film plane will result in a camera that produces a thin strip of in- focus image in the middle of the photograph.

The amount of curvature required depends on the design of the lens.

 

[MattKing]

Nicholas et al:

Correct me if I'm wrong...

Spherical aberration arises because lenses are (or were) ground with spherical surfaces, rather than the ideal, which would have been parabolic surfaces. If they are ground (or molded) with parabolic surfaces, they will render flat surfaces sharp on flat film.

One uses a curved film plane, because one wants to have a sharp image on the film of a series of items that are all equidistant from the nodal point of the lens (as in a Cirkut camera).

Matt

 

[keithwms]

Well, I haven't thought about this recently, but I suppose that the main virtue of a curved plane is that you can avoid coma and astigmatism correction. I.e., you can use much simpler lens designs without need for those corrections. If you used a curved film plane with a corrected lens then I don't think the result would be good.

But the film plane we're talking about, it's only concave in one direction, right? Unless you put liquid emulsion on a suitably sculpted piece of glass with the right radius of curvature, and then enlarge from that, I don't see much benefit.

Consider the eyeball.

 

[alanrockwood]

Warning: Scientific and somewhat rambling discussion follows. The field curvature of a lens is very easy to calculate IF you know the refractive index of each lens element in the design, along with the curvature of the surfaces of each lens. The formula is known as the “Petzval sum.” One can design a lens with zero curvature of field simply by making the Petzval sum zero.

Actually, there are some additional subtleties here. When I say “zero curvature of field” in the above discussion I really mean that the Petzval sum is zero. Unfortunately, curvature of field and astigmatism are closely related an interdependent. If we define the curvature of field as the surface intermediate between the tangential and sagital surfaces in the stigmatic image then you can have curvature of field when the Petzval sum is zero. However, it is impossible to correct a lens so that both astigmatism and field curvature are fully corrected without making the Petzval sum zero.

Now that I have managed to confuse everyone let me suggest some software that can let you play with lens designs. The software is called “WinLens,” and the basic version of the program is a free download.

Finally, to get back to your original question, unless you have information on the lens design as discussed in the first paragraph there is no way to calculate the curvature of field. It might be possible to measure it if you have the right kind of experimental setup, but this would not be easy unless you are well equipped and knowledgeable.

Finally (I know this is the second “finally” paragraph), in theory you could grind a ground glass focusing screen that is curved, but it is probably simpler to just use a flat focusing screen and stop the lens down to minimize the effects of field curvature on the sharpness of the image. Also, any curvature in the film and focusing screen would be a compromise. If you bend it in one direction to compensate for field curvature, you leave the other direction, the one that is not bent, uncompensated. This means that you should bend it in the long direction, which is what I assume you have in mind, and not bend it in the short direction.

 

[AgX]

Keith,

the idea of a bent film plane was to correct for a spherical (or at least not flat) image plane at least for one dimension, the longest side of projected image.

 

[keithwms]

AgX, so is the idea to use that partially curved surface with an uncorrected lens? It seems to me that using a corrected lens with a partially curved plane will only compound the errors.

Jeff, Alan, how about setting up a (simple, uncorrected) lens and using some paper to find the best curvature by projection?

 

[ic-racer]

I presume you are using a curved film plane for some special visual effects. You can set it up so that the center band can focus at infinity (and the edges will be blurred if there are no near objects at the edges) or you can set it up so that the two bands at the edges focus at infinity and the center is always blurred. You would do either by setting the distance from the film (center or edge) to the lens mounting flange so that it equals the "Flange Focal Length" of the lens.

 

[Jeff Searust]

Heres a site with some good pix of an Agfa Chief where you can see the curvature of the film plane.

http://www.foundphotography.com/PhotoThoughts/archives/2007/03/agfa_chief_120_mod.html

What I am working on is basically a wood box with the a lens on the front and a door that opens in the back. Think of it as a wee sized pano camera. 6x9,.

 

[Maris]

This is how I measure field curvature of lenses.

First mount the lens on a view camera and focus a point light source at the centre of the ground glass. Measure the distance between the film standard and the lens standard in some convenient way. I use a digital vernier caliper.

Then turn the camera so that the image of the light source now falls at the edge of the ground glass. Focus again. Measure between the standards again using the same fiducial points as before.

The difference between the second and first measurement is how much the image "surface" bends away from a plane. If you know the focal length of the lens then this "bend" gives you enough data to calculate the radius of image curvature by good old Euclidian geometry.

Alanrockwood's warning about astigmatism is very pertinent. A stigmatic lens (any simple lens, for example) will give two focus distances at the edge of the ground glass. In one setting the point source will focus as a short vertical (tangential) line. In the other setting the focus will be a short horizontal (sagittal) line. Take both measurements, make an average, and call the result the "image distance".

 

[Jeff Searust]

This is how I measure field curvature of lenses.

First mount the lens on a view camera and focus a point light source at the centre of the ground glass. Measure the distance between the film standard and the lens standard in some convenient way. I use a digital vernier caliper.

Then turn the camera so that the image of the light source now falls at the edge of the ground glass. Focus again. Measure between the standards again using the same fiducial points as before.

The difference between the second and first measurement is how much the image "surface" bends away from a plane. If you know the focal length of the lens then this "bend" gives you enough data to calculate the radius of image curvature by good old Euclidian geometry.

BINGO