Curved Focal Planes and Raytrace

[Ralph Barker]

Although a bit late to the discussion, Father G., I'm confused about your choice of roll film instead of large format film - at least 4x5, but perhaps larger. Depending on what you'll be doing with the negatives, a larger film size would seem to simplify many of the issues, including the lens resolution spec you're looking for. On 4x5, for example, 75 lp/mm is about the same as 150 lp/mm on 120/220, while 75 lp/mm on 8x10 would be equivalent to 300 lp/mm on 120/220, when each is enlarged to the same sized print.

For extremely high resolution lenses, you might explore contacts with NASA or other government agencies (or, the lens manufacturers directly) to see if you could borrow something that would work in your application. (Think in terms of the stories of the CIA spy satellites being able to read the date on a dime from space orbit.)

[FatherGerrit]

Hi Ralph, your questions are good and ones we had to answer. We are using roll film, specifically microfilm, because it has resolving power much higher than most available sheet film. Tech pan is the highest that is readily available, with a max resolution in our contrast range of 100 line-pairs/mm - we need more. This Imagalink microfilm has over 500 lp/mm in our contrast range. But the biggest reason is cost. To use ester-based holographic film, such as Slavich’s PFG-03C panchromatic, which would more than give us our needed resolving power, would cost $15,000 and doesn’t have good grey-scale (we need full 16 byte - PFG gives us only 4 byte). To use Kodak’s Aerecon (a much better film than Imagelink, and would give us a 9.5” image and good grey scale, but also only in 200’ rolls) for this project would cost more than $7,000 in film – while the Imagelink is only $600! As far as help from NASA and the government is concerned, they have specific prohibitions from working with religious institutions – we wouldn’t want your tax dollars going to further our religious work if it were in conflict with your faith or absence of it, would we?

[Graeme Hird]

Why not spend $7000 in film? Surely the oldest book in the world is worth a small investment in the best quality available.

Still not swallowing this ......

[Bob._3483]

If a Troll, it is at least one of the more entertaining ones... The yahoo.com email address is a bit suspicious too - and Google can't find a trace of it being used before now, but a quick Google does show there were bibles of a sort compiled as early as AD 150....

Personally, I always thought Pascal had it about right...

Cheers,

[Nick_3536]

What happened to the Agfa repromaster lenses that the camera likely was born with? Ya they are slower then the ones you've bought but since they need to be stopped down to F/22 wide open speed isn't an issue.

[FatherGerrit]

Hi Nick, The whole point is NOT to stop down to f22. At f4, one can resolve 400lp/mm, at f22, the resolving power drops to 73 lp/mm. We tried the Repromaster lens – it didn’t meet our requirements. We have worked with some of the best in the field, and I came here because many of you are also tops in the field, but I have no other way of asking you all. I really don’t need alternatives, I just need to plot the characteristics of what we have. About spending $7,000 for film, if I had it, I would. Unfortunately, I am funding this work myself, and simply can't afford it. Our church spends its limited resources on people in need, not books. As far as the doubters are concerned, I am not asking for anyone to believe anything I have said. I only asked a simple technical question, and need only a technical answer, requiring no belief nor validation on anyone's part.

PLEASE, MAY WE STAY “ON-TOPIC”?

[Michael S. Briggs]

The differences in the quality of the various steps, and how strange some of your choices are, is what is making us wonder whether this is for real. If it is for real, with the effort and money you are willing to spend to achieve extreme resolution etc., I think you consult some experts in person, or probably hire them. Think of your reproduction process as a chain connecting the original to the final copy: to use a cliche, the chain is only as strong as its weakest link. The quality of the final copy will be largely determined by the weakest step in the imaging chain. I think you are making extreme efforts to strengthen some links of the chain while mostly neglecting others.

Your great concern with flatness of the film is probably misplaced unless you flatten the original, which I would think would be inappropriate for a book of such antiquity. This is a large format forum, so perhaps the advise will be biased, but one usually obtains higher ultimate resolution by going to larger formats. The reduced enlargement from film to final copy more than makes up for the reduced resolution on the film. Film flatness becomes less of an issue in larger formats because of the higher f-number typically used. You will need to stop down some anyway because your subject won't be perfectly flat.

Using Pyrocat semi-stand development seems to me to be a very strange project like this. You you sure that you can't get the results that you need using continous tone film like T-MAX 100 and a more conventional developer?

You should probably conduct some experiments with the lenses. For example, just because Schneider recommends f22 for G-Clarons for a particular use doesn't mean that a wider aperture won't work better for you -- it depends on the size of the image that you want. Some of the lenses you mention are very unlikely to give you the copy quality that you want, e.g., Aero-Ektars and Wollensaks. The first because it is designed for the distant objects, and both because of their age. If you want the best, you should buy a current top-of-the-line lens for designed for copy, macro, or enlarging. Besides some that have already been mentioned, there are the shutter mounted Apo-Macro-Sironar, Macro-Symmar-HM and AM-ED Nikkors lenses, and then the Apo-Componon-HM and Apo-Rodagon-N enlarging lenses (not in shutter). Lens design and manufacturing has improved with the years, and if you want the kind of resolution you seek, you should use on of these lenses. Getting back to the original question, these lenses are designed to image flat objects to flat film and should have minimal image curvature. I think it unlikely that you will gain much with a curved film plane, particularly without a perfectly flat subject. You will need to experiment with which aperture delivers the best resolution for the size image (film) that you will be using for the typical flatness of the subject.

With these modern, currently made lenses, technical performance info is more available than for old lenses, particularly from the German manufacturers. Also, the manufacture might be willing to consult on the use of the lens for your special project, e.g., to recommend the best aperture for a particular reproduction ratio and image size, or to estimate the resolution the lens will delivery. Starting from the resolution that the lens will give you, you evaluate the film. There is no point in using a film with much higher resolution than the lens will deliver.

[Mark_3632]

Using flash on a document, any document is a bad idea. Speaking from experience. I was not allowed to photograph a 150 year old treaty with flash. only low watt balanced continuous light, as directed by the national archives of the United States, and your tax dollars paid the 100,000 dollar final cost to house the treaty for a couple of month display. This was the Treaty of 1868 That was between the Federal Government and the Navajo Tribe. I would think that a manuscript as old as the one you are intending to photograph would be protected by similar if not more harsh restrictions. What institution is this book housed in again?

You are not standing it on end you are opening it only half way, putting less stress on the binding than having it open completely. Remember someone is holding it open while you run the camera. ALso what is the vacuum easle for if the book is bound?

Now I'm curious

[Michael S. Briggs]

"The whole point is NOT to stop down to f22. At f4, one can resolve 400lp/mm, at f22, the resolving power drops to 73 lp/mm. " I suggest it would be a mistake to design your work around the figure of 400 lp/mm. That is the theoretical value on-axis. You are not going to get that from any of the lenses within your budget. Some special purpose lenses may approach this response, but only with severe limitation, e.g., image sizes of a few mm, or very narrow fields of view, or for monochromatic light. Telescopes for astronomy can be diffraction limited, but their field-of-view is typically less than one degree. Commercial lenses suitable for imaging to 35 mm or larger won't reach any near 400 lp/mm. Even if you could obtain a lens that performed at the theoretical limit, the resolution falls off-axis.





For some indication of realistic resolution figures, take a look at the lens test results of Christopher Perez and Kerry Thalmann at http://www.hevanet.com/cperez/testing.html.
There results are not exactly applicable to your application since they tested lenses for photography of distant objects, but it gives an indication of actual performance of lenses. Even though the widest aperture of most of their tests was f11, most of the lenses improved when further stopped down, particularly when the figures for the off-axis measurements are considered.





Here's a budget suggestion: one seller on ebay is frequently offering surplus 150 mm Apo-Rodagon-N lenses at prices much lower than the normal version. Their version must have been made for an application like a copying machine -- it lacks an adjustable iris -- but you can install a metal disk with a hole of the optimum diameter -- the seller suggests f8. You will also have to rig up some way to control the exposure, perhaps a Packard shutter.





The idea of making a curved film back supposes that field curvature is a dominant or at least major defect of the lens near wide open. This may not be true. One simple experiment would be to focus at the center and take a photo, then focus in the corner and take another photo. If the resolution in the corner is signficantly better in the second photo, then curving the film will help; if not, then it would be a wasted effort. Ray tracing is usually done by calculation on a computer- -- is this what you had in mind? There are some freeware or shareware ray tracing programs, but you will need to know the complete optical perscription of the lens, including dimensions and glass properties. In a some cases, such as the Aero-Ektar, you can get this from patent data (but the Aero-Ektar is a poor choice). You could try rigging up an optical bench and measuring the position of best focus at various angles off axis, perhaps imaging a light bulb filament or point source onto ground glass and then measuring the focus position with a dial indicator. You will probably be better off not to spend the money on CNC machining of a curved film holder and instead spend it on a modern macro/copy or enlarging lens and use flat film.

[Graeme Hird]

Fr Gerrit has responded off-forum to my email. He appears to be serious about this project, and I apologise for casting aspersions about his authenticity.

Fr Gerrit,

Something I noted in my post was the depth of field issue. Shooting at f5.6 will give you a very shallow depth of field, which actually translates to depth of focus on the film side of the lens. At f5.6, the slight variations in the flatness of the book (which could be more than slight, if the book is bound in any way) will throw the focus out much more than any variation in the flatness of the film could. You need the depth of field associated with the smaller apertures to maintain the focus throughout the image. What this means is that although you might have higher resolving power at f5.6, much of your image will be out of focus and thus not resolved as effectively as at f22 (for instance).

You have noted that you are concerned with the flatness of the film and are concerned with film movements of less than a tenth of a millimetre. By stopping the lens down, film flatness (or lack thereof) is not such an issue, since larger variations will still fall within the depth of focus.

To further your research on this, I suggest you look into the reprographics used to produce printed circuit boards. They need extremely fine resolution to produce small circuit boards, so find out how they do it.

In the end, you will most likely come back to the large format films and modern lenses that people here have been expounding. There is good reason why the industry standard is a large film and a process lens stopped down to optimal aperture.

If that is too expensive, your next option is to hire a digital back on something like a Sinar.

Regards,