Excellent - Thank you for the clarification, and for the additional information.
Those of us who like to have a basic understanding of these matters, are grateful that there are real scientists and mathematicians on the forum.
Question: What happens when we scan film ? Can we retrieve some of that "vanishing" data ? Can we have our cake, and eat it too ?
Last edited by Ken Lee; 29-May-2008 at 05:36.
Question: What happens when we scan film ? Can we retrieve some of that "vanishing" data ? Can we have our cake, and eat it too ?
This is probably the most irritating question since scanners became available for the amateur !
Clearly, good amateur-grade flatbed scanners cannot retrieve everything which is recorded on film, but do we actually need to extract everything ? Scanning beyond, says 3000 samples per inch does not usually bring any additional sharpness with amateur-grade flatbeds. So the larger the film size, the better in this case.
Drum scanners on the other hand can do something excellent but I have no idea whether those machines are still manufactured, although many are still in use.
The problem with scanning film, even if your machine like a drum scanner can reach a real optical resolution of 10,000 samples per inch, is that you cannot easily get rid of granularity when your scan analyses the surface with a very small spot. The problem was well-known in the past with micro-densitometers, density fluctuations on output increase when your analysing slit gets smaller. I do not see how a drum scanner could bypass this fundamental effect !
Your are stuck exactly like with a image photon detector with a limited quantum efficiency : you have to merge pixels together and average the reading in order to decrease the noise.
So you have to find a trade-off between noise and resolution, the good approach is to use large format film for which an amateur-grade scanner will deliver acceptable results. Then, the issues of optical resolution and film resolution usually become less important than final scanner effective resolution !!
Wonderful - Thanks !
There are at least three manufacturers still making drum scanners. Screen in Japan, ICG in England, and Aztek in the USA.Originally Posted by Emmanuel BIGLER
There are no scanners that I know of that can optically scan at that rate. The highest seems to be an Aztek Premier with a 3 micron aperture which tested out at 7264 ppi. But this test was run by the late Phil Lippencott who was the president of Aztek at the time he made the study. I'm just saying there's the possibility of a conflict of interest there, so make of the study what you will.
Oh, yes, the study is at scannerforum.com, follow the DIMA 2002 Scanner Roundup link, look at page 22 for his final results. Note also the lack of testing of the big Screen scanner which might do a bit better than the Premier. But we wont know until somebody runs the tests and publishes the results. Sigh...
The thing is that a PMT is a much better photon detector than a CCD is. I've been drum scanning a while now and I have yet to see anything I would classify as noise from any scan I've made. The weird thing is how much better performance the PMT's have over CCDs in the least dense areas of the film. I would have thought that they would be nearly equal here. But PMT's pull considerably more information out of the "shadows" of a B&W negative than the CCD scanners I've used.
There's something else going on here too. Specifically, scanners are deterministic devices. They lay a highly repeatable "virtual grid" over the image and sample the image through the holes in this virtual grid. The film however is stochastic in nature. Density fluctuations across the film are nearly random. The film grain itself is very small, but the emulsion is 3D in that it has thickness. It is this thickness which allows grains to overlap and form grain clumps which are big enough that a scanner can see them.
For more about how film forms an image, what the image actually looks like at the detail level (excellent illustrations), and what levels of resolution various films can achieve, see the excellent Tim Vitale paper that is so amazingly informative. Tim does very good, very thorough work, and explains it all so much better than I can ever hope to.
The range of sizes of these grain clumps is very large, while the scanner looks through fixed size holes in its virtual grid. There is little hope therefore of ever imaging a film grain clump, because there is little hope that the film grain clump is the same size as the hole in the virtual grid. And if it were, there's even less hope that the film grain clump is centered precisely in the hole of the virtual grid.
What I'm saying here is that the scanner does not image the film grain. What it does is look through it's virtual grid and sample the film at that point. That this works at all indicates that the image information lives at a level considerably higher than that of the film grain clump. That is, it takes a whole lot of grain clumps to make image information -- it takes a lot more than just two grain clumps to record the edge of a tree branch for example.
The bottom line here is that scanning is almost certainly going to be the "bottle neck" of resolution in a lens/film/scanner workflow. If for no other reason than the problems of looking at a stochastic medium with a deterministic tool.
Bruce Watson
Emmanuel and Bruce, thanks very much for your input - as Ken mentioned, it is nice to have folks like you on this forum.
Thanks very much to Bruce for the information about drum scanners still being made.
If you arrange side by side square pixels of 3 microns square, this makes 8400 pixels per inch so it is nice to see that the instrument could get very close to this limit.
Actually if you look at the theoretical sampling limit for a square aperture, you could still gain something by moving the slit by one-half of its width, i.e. taking about 16,000 samples per inch for a 3 micron slit. This "doubled sampling" of course is not possible with a modern scanner in ther direction where sensors are arranged in line side by side, however in the scanning direction, this is perfectly possible, and as far as I know, actually used.
Actually in the eighties I have worked with the ancestor of modern scanners, it was a computer-controlled micro-densitometer made by Perkin-Elmer.
This was not a drum scanner, this was a high precision X-Y stage with a repeatability in the micron range. The instrument had the advantage over drum scanners of handling glass plates that many scientific labs still used at the time.
The smallest slit was 5 microns (actually, in a microdensitometer the slit is the projected, de-magnified image of a mechanical slit) and as a student working with Kodak high-resolution plates at the time (Kodak HR type 1A, similar in granularity and resolution to the legendary 649F spectroscopic plates, but non-chromatized, blue-sensitive only).
You bet that I rushed on the instrument pushing it to its limits : too bad, scan times were very long and mass storage was only on tape...
So I had tried to apply my university course immediately and tried to sample at 10,000 samples per inch (5 microns side by side would make about 5000 pixels, the extra factor 2 was to comply with Shannon's theorem)
Quickly I had to work with a modest 20 micron slit in order to get tractable files ! And there was a huge users queue, so scanning time was money !
About noise, I was not referring to electronic noise but to density fluctuations due to the random nature of sliver grain patterns, known as Selwyn's law, which still serves today to define RMS granularity. I have no idea about the RMS granularity value for Kodak high resolution plates.. too bad since I have done so many measurements !
So I doubt that any top-class scanner could bypass Selwyn's law, but I do not know, the issue in itself is fascinating !
The Perkin Elmer machine I know from the past used a photo-multiplier, but I have read that recent silicon sensors can now exceed the performance of a photomultiplier to some extent ; a phomultiplier has a quantum efficiency of about 40% (limited by the photo-cathode) whereas top-class silicon sensors can exceed 80%
However the photomutiplier is still in use for special applications.
There have been many valuable contributions here but I have not seen a simple answer given for the original simple question.
The simple answer is that you cannot get there.
Either equation, the 1/R^2 version or the 1/R version suggested by Struan (Thanks for that by the way.), implies that the resolution of the combination will always be less than that of the lens. If your sensor is much, much better than the lens, then the resolution of the system can be arbitrarily close to that of the lens, but never quite as good.
Of course equations are just models of reality, and I always remember a pragmatic old engineering professor's disparaging joke about mathematicians, so don't let this theory keep you from trying. Better is the enemy of good enough.
- Alan
Equations aside, if one wants to know the total resolution of a combined system, be it digital sensor or film, it is easy enough to test. Just obtain a suitable target, put the camera on a tripod, select the best aperture for resolution (or bracket if in doubt), focus carefully and trip the shutter. You can determine resolution of digital files directly on your computer monitor, for film you will need a microscope.
Sandy King
Just obtain a suitable target, put the camera on a tripod, select the best aperture for resolution (or bracket if in doubt), focus carefully and trip the shutter. You can determine resolution of digital files directly on your computer monitor, for film you will need a microscope.
Yes, Sandy ; last year I had the privilege to play this game in the digital version with a good photographer friend of mine. So the testing team was made of a professor (only to control the theoretical aspects of the game
We borrowed a superb piece of equipement from a professional photographer equiped with top-notch "digital" Rodenstock lenses and a digital back.
In the collection of lenses I inserted a 2.8-100 Zeiss planar from the sixties (the same fitting the baby linhof) an we checked the results after enlarging files on a good professional screen.
The game is much faster that the equivalent with film, but so far the conclusions are : if your focus is OK, you reach the theoretical limits of the sampling theorem based on the sensor pixel pitch. In this case, the theoretical limit was 66 cycles/mm, most files reached 60 cy/mm with the best f-stop with almost all lenses provided that the focus was OK.
For short focal lenghts in addition to various f-stops we did some focus-bracketting and selected the best file.
I was a bit dissapointed, since I had theoretically computed that my planar, probably the same design as the Rolleiflex's, but scaled by a factor 100/80, should resolve about 96 x 80 / 100 = 76.8 cycles/mm, as based on Chris Perez's extraordinary figures for various MF cameras (including the record of 120 cy/mm for the 80mm lens of the Mamiya 7)
http://www.hevanet.com/cperez/MF_testing.html
Too bad
I have also tested the Mamiya 7 lenses, including the 80mm. Unfortunately I can only get about 85 lines/mm with my 80mm lens, a far cry from the 120 lines/mm recorded by Chris Perez.[i]I was a bit dissapointed, since I had theoretically computed that my planar, probably the same design as the Rolleiflex's, but scaled by a factor 100/80, should resolve about 96 x 80 / 100 = 76.8 cycles/mm, as based on Chris Perez's extraordinary figures for various MF cameras (including the record of 120 cy/mm for the 80mm lens of the Mamiya 7)
http://www.hevanet.com/cperez/MF_testing.html
Too bad
Your experience with the digital sensor is not surprising. From some testing with friends it appears that an optimistic best resolution for the P45 MF digital back is about 62 lines/mm, quite a bit below the 73 lines/mm that would be predicted by pixel count alone. That is in the center only as we did not test the edges.
Sandy King
Last edited by sanking; 30-May-2008 at 08:59.
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