Curved Focal Planes and Raytrace

[Brian Yarvin]

Wow! This is an interesting thread. I'm no sharpness expert (as those who've seen my work will attest) but I do know about some people in Europe who've dealt with the same problem. Note that the claimed resolution is over double what's being sought here. Here's a link:

http://www.kigamo.com/scanback/dmc.html

BTW...my opinion is that Father Gerrit could get the image quality he wants with a Polaroid MP copy camera, a specialized document holder, a modern copying lens, and a capture device like a Kigamo 8000XP or the BetterLight unit that uses the same sensor..

Of course, such a solution wouldn't be much fun.

[Al W]

"Steve, we are aware of the coatings for color work, but we are doing 3-color separations and focusing for each wavelength,"

For three color separation you set the focus and don't touch anything between shots. If you have a lens that can't focus on all three colors at once, you have a lens with different focal characteristics for each color. "Focusing for each wavelength" will only exacerbate your problem by creating reduction scale changes. Alignment will be impossible in this case. Better to get a lens that can do the job you want done. You want a lens that is sharp and aligned for all colors corner to corner.

From the beginning photo to the final print there will be a tolerance error building up. You need to reduce it at every step of the way to get the product you want. You seem to be willing to spend a lot to get perfect focus, the lens is where your money should be directed.

[Kirk Keyes]

In addition to the reason listed right above by Al W as to why you do not want to refocus when doing color separations, the idea about having a curved film plane to correct for field curvature will have major issues as well.

It seems to me that by making the film curved to "match" your lens' field curvature will cause problems with the reproduction ratio. Even though the Father wants to have this book copied with very high resolution, he seems to be completely forgetting about orthogonality(!), i.e. preserving length and distance relationships of the objects in the image. Small changes in the film plane will cause distortions in the final image when photographing flat objects. One part of the image may be reproduced at say 1:1 (as a simplified example) while other parts could be at 1:1.1 or 1:0.7 or worse... Ratios may get even worse with the image being copied onto 220 film, versus 4x5, 8x10 or larger. This will not be a good thing.

Also, Father, why can't longer exposures be used to compensate for mildly stopped down reprographic lenses, such as a modern G-Claron? Is it simply for time?

Perhaps you need to postpone this project until more money (only $7000! to do this) can be raised to do this project correctly (which will probably mean copying onto an Estar-based film (for dimensional stability) using a large format copy camera.

Kirk

[FatherGerrit]

Hi All, Thanks again for your insights. Al and Kirk made important points. We think we have solved our dilemmas thanks to suggestions from the forum, both on-forum, and off. As a result for your insights, we will be testing, in our system, some more modern lenses and using a vacuum-backed flat film plane (our camera already does this by design). A forum participant also referred us to a photo shop that has dated Aerecon film (it has been frozen since acquisition) for sale at a price we can afford. We will now be able to use 7.5” X 9.5” film format on Plus-X Aerecon. This is very different from regular Plus-X as it uses flat-grain technology (like T-max), has a wider latitude, extended red sensitivity (super important for our application) and an Estar base. It has less resolving power than we had planned on, but the huge negatives make up for that (see guys, we actually do listen to the large-format mantra). Therefore, we will have about 4 times the image to work with, as compared to the 4 X 5 we had planned on. Yes, bigger is better! The film is from Mr. Photo (mrfoto@localnet.com) and Ed, the owner, gave us a great price! We are getting this wonderful film at about $0.17 on the dollar in 9.5” X 125’ rolls. If you don’t mind cutting your own film, this is a super way to go. They also have 5” wide films. I highly recommend them. About the “registration” for color separation, as Al pointed out, this is critical. Fortunately for our application (not printing, but going direct to the drum scanner/computer) our wonderful forensic software from Reindeer Graphics (http://reindeergraphics.com) corrects for all that. It automatically registers every image regardless of size or magnification. It will even correct for much of the lens variability we may still encounter. We are still investigating a better lens (more modern), based on the forum’s suggestions. So, with all your help and info, we have the challenges solved – Thanks!