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Thread: Curved Focal Planes and Raytrace

  1. #1

    Curved Focal Planes and Raytrace

    Thank you, All, for your sage advise regarding small-format lenses in a large-format application! We chose to take your advice, and stick with large-format lenses for our imaging of the oldest known New Testament manuscript that we are working on. We have purchased some lenses, (An Aero-Ektar (f2.5), some Wallensaks (f4.5-5.6) and Tasser-types (f5.6)in the 150mm-210mm range). We have also consulted the Astrophotography community where their systems routinely yield extremely high resolving powers. They have emphasized the importance of matching the curvature of the focal plane to the lens. Does anyone know where a raytrace of an individual lens can be performed? We can then use this information for custom-making of a curved focal plane for our camera (We are modifying an old Repromaster Copy camera). Thanks Again for all your help! Fr. G.

  2. #2
    wfwhitaker
    Guest

    Curved Focal Planes and Raytrace

    Most lenses intended for copy work are flat field lenses. They are designed specifically to translate the flat field of the work being copied to a flat film plane. In fact, most copy cameras use a vacuum back to insure that the film is in full contact with the flat (reference) platten. To use a curved film plane would only result in most of the image being out of focus. Unless I misunderstand you, you will achieve your best results using an accurately aligned copy camera with the object to be photographed and the film being held dead flat on parallel planes.



    The astronomical community makes use of some often extremely esoteric optical hardware and in some cases those systems do used curved film planes. Astrophotography is extremely demanding in that points of light must translate directly to points on the negative (or imaging device) and with the spatial relationship completely undistorted. But astrophotographical systems are typically relatively narrow field devices while copy systems are fairly wide field. Copy systems also operate at much greater magnifications (shorter lens-to-subject distance) than astronomical systems which work essentially at infinite (heavenly!) distances.



    If I understand your needs correctly, your best bet, it would seem to me, would be to contact someone in the graphics or publishing field who has experience making high quality reproductions or plates.



    Best,
    Will

  3. #3

    Curved Focal Planes and Raytrace

    Thank you Will, we share your concerns. In fact, we have intended on using all you have suggested, with the added concern as to whether or not the "flat lens" truly is flat. Since we found the well documented observation that a deviation of only 0.08mm (about 0.003 inches) in film-flatness causes a 49% decrease in contrast, we wanted to make sure what we were dealing with. We would profile the film-back to follow the deviations of the actual lens Raytrace. That is why we wanted to plot the actual performance of our lens, ergo, the desire to do a Raytrace on the various lenses we have - but we don’t know where to have the Raytracing done. BTW, we found a wonderful material to machine a vacuum-back out of. It is called Metapor. This aluminum material is porous. It is used to make smooth vacuum-form molds for precision plastic film-moldings. With this material, and a plot of the lens Raytrace to program a CNC mill, we could have a precision vacuum-back follow the exact profile of the lens. So, we have the technology to deliver a high precision back, we just need the Raytrace to determine the final shape. Thanks for your wise suggestions.

  4. #4

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    Curved Focal Planes and Raytrace

    Father Gerrit,

    I'll second Will's advice. I don't think anyone knew that the project was to copy flat mauscripts. The repromaster should be a good camera for what you're doing but it is desirable to use apochromatic lenses for copy work, which I suspect your lenses are not. Apochromatic lenses focus (at least two) different colors of light at the same plane, or at least at the same reference point, improving image quality. Typical lenses designed for copy work would be the Schneider G-Clarons, Repro Claron, Goerz Apo-Artars, or Rodenstock Apo-Ronars, and I would expect your repromaster might have originally came with one of these. Maybe you can use a barrel lens (one without a shutter), and good examples of the above lenses in barrel can be had for $150- $200.

    Thanks!

    Steve

  5. #5

    Join Date
    Oct 2003
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    390

    Curved Focal Planes and Raytrace

    Is it illuminated? I love illuminated religious texts. So much love went into the drawings.

    To flatten the pages to get a flat field of view place a piece of archival board under the page you are shooting. Keeping the book at a right angle instead of open flat will keep the pages from arching in the middle. I used to copy historic text and photographs for slide shows and publication with LF equipment. This works. It is possible to light the text evenly when the book is like this too. Since you are copying text it is of utmost importance to align your copy camera and the base so both are perfectly parallel.

    Photographers are generally nice people as are people in the graphics industry. I bet one would come down to help set things up. I would do it just to see the book.

    Good luck

  6. #6

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    Curved Focal Planes and Raytrace

    Not to be insulting or anything, but why did you get all those lenses that aren't designed for the task you want to accomplish? If you have a Repromaster camera, get the lenses that go with it, they'll do what you want better than any of the lenses you have. AeroEktars, for example, are optimised to a fare-thee-well for infinity. They're not meant to be used as copy lenses..

    And, not to pick a fight but because its news to me, where did you get "the well documented observation that a deviation of only 0.08mm (about 0.003 inches) in film-flatness causes a 49% decrease in contrast"? If this were true, roll film cameras would be unusable.

    As for making your own vacuum back, you'll probably to better do find a graphic arts camera that has one.

    Cheers, good luck,

    Dan

  7. #7

    Curved Focal Planes and Raytrace

    Wow, thanks for the swift and expert responses. If I may respond one at a time? Yes, Steve, we are aware of the coatings for color work, but we are doing 3-color separations and focusing for each wavelength, individually (we are using dichroic filters for red, green and blue separations, so we have lots of light to focus with, and a dissecting microscope for the actual focusing on the etched, not ground, glass – Bill Maxwell’s screens.). We have a nice G-Claron, but it works best around f22, according to Schneider. That would yield a max resolution of only 73 line-pair/mm. We are shooting for twice that (we have some very small details that need to be resolved for the forensic work we are doing), which means we can’t go smaller than about f8, preferably f5.6 (and, of course we don’t want to be at full aperture of the lens because of all the artifacts of using a lens wide-open.)

    Good question Dan, and we never have any reason to feel insulted – you guys are gems! – we got all copy versions of the lenses, except the Aero-Ektar. Even the Aero-Ektar was praised because it has such a flat-field. However, we got it because it was so fast, that we could stop it down a lot. Besides, it was only $25, so we had little to loose in trying it. Dan, the film-plane flatness issue is covered well on the Zeiss site, and many others. Roll film is a real problem, especially when the film does not have a time to rest in the flattened state, such as with rapid advancing. In fact, Zeiss recommends only 220, instead of 110 when using roll film, because of this problem. Our film will actually be from roll film stock (Kodak Imagelink HQ, 3,000 sheets of film hand-cut to size, because of its amazing resolution, and we will be using Sandy King’s Pyro-cat HD, in a semi-stand developing configuration to improve sharpness and grey-scale – similar to SPUR’s methods), but it will be stored flat, under pressure for a week before use. Our planned vacuum-back will handle the rest, if we can find a company that can do the Raytrace.

    Nope, Mark, unfortunately it is not illuminated - I share your love for them. They didn’t illuminate Bibles in 165 AD. And YES, the people in the graphics industry are wonderful – in fact, they gave us the Repromaster for free (Edgewood Press, Inc, Covina, CA – thanks Ernie!). We have designed a special book cradle with page-backing that helps to flatten the pages (they are very ancient parchment and very convoluted) and to prevent the writing on the back side from showing through the parchment. The manuscript is too fragile to stand it on end – we must have it laying flat.

    I must say, you all are wonderful – we can’t begin to thank you all for your interest and good advice! Blessings, Fr. G.

  8. #8
    wfwhitaker
    Guest

    Curved Focal Planes and Raytrace

    Fr. G.,



    Well, it sounds like you're already way above my head. However I'd like to add a couple more thoughts. Raytracing, as I understand it, is a mathematical process rather than a physical one. Given all the lens parameters (surfaces, spacing, glass data, etc.), one can plug that information into an optical design program (many of which are readily available for your PC) and arrive at some theoretical estimation of lens performance. But it sounds like you wish to take an actual lens and physically analyze its performance. Some sort of an interferometer (Zygo, etc.) is probably what you want. I would suggest contacting a precision optical firm (possibly consult the Optical Society of America for a referral). It won't be cheap. Similarly you might consider making contact with someone at the University of Rochester or the University of Arizona (Tucson), both of which have extensive optical programs. Sooner or later you'll find the right person who can help you directly.



    Still I'm not entirely clear on what your ultimate goal is. I still wonder if perhaps a good Artar or Apo-Ronar in a vertical copy camera might accomplish most of what you need more economically.



    Best,
    Will

  9. #9

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    Curved Focal Planes and Raytrace

    Call me an agnostic, but I smell a troll. A book from 165 AD? Even if such a book existed, would something so priceless be entrusted to someone who does not have previous experience photographing documents?

    You want to photograph pages in a book and are worried by 0.08 mm of variation in flatness of the film. Yet you want to use f5.6 on your lens, where depth of field, not to mention resolution, are at their minumum? Perhaps you have a way of altering optical physics. You want 150 line pairs per mm? Better ask for devine intervention.

    Come on people - think about what Father Gerrit is asking. He's giving enough clues!

    Graeme

  10. #10

    Curved Focal Planes and Raytrace

    Thanks Will, that is new information to me. I thought Raytracing could be done both directions - to an existing lens, but I see your point. The people at Rochester Institute of Technology helped us with the initial part of this project a year ago, and we should obviously see what they suggest about this. I forgot to answer some questions and suggestions. The first is using the existing vacuum back. It works fine, for flat fields, but to try to correct for lens uniqueness, it can't do it. That is why we considered a custom vaccum-back. It also requires loading and unloading of the film for each exposure in the dark. We have 3,000 images to take - that is a lot of turning off and on the lights. Since it takes 30 minutes for our special lights to stablize, it would be 1,500 hours of just waiting for the lights to come to stability. One of Dan's questions was - why don't we use the lens that came with the camera? We started with the Repromaster lens. It is good when closed down, but to get the fast aperture we needed, we had to open it wide. Then we got what is called a Petzval field curvature. The light actually goes though the very edges of a wide open lens and causes a spike in the focal plane, making a spot in the center of the image way out of focus. When we stop the lens down to f16, where this problem goes away, it decreases our resolution to 100 lines/mm - too low for our needs. And yes, we are aware of the dangers of the Aero-Ektar, and have taken good shielding precautions, not only of our selves, but also of unexposed film. BTW, the yellowing goes away when the lens is left in the sun for a week. As far as Graeme's comments are concerned, resolution is at its maximum at small f stops and decreases as you close down - that is basic optics. Photonet has a great tutorial on lens optics. The flatness issue can be easily verified by going to Zeiss, and many others with a simple net asearch on film flatness. As to my qualifications, that is not on topic for this forum. If you want to challenge me personally, you have my email address, and you are welcome to use it. Blessings All, Fr. G.

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