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Thread: Resolution limits of prints

  1. #11
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    Resolution limits of prints

    Paul -

    Ctein's paper-resolution experiments were conducted by contact-printing a glass bar target, using sufficient pressure - "equivalent to contact-printing an 8x10 negative under several hundred pounds of pressure" - to insure good contact with the paper, and exposure tuned for optimum resolution. He describes the bar target as having "nearly 100% contrast out to about 100 lp/mm; thereafter, contrast falls off until it reaches zero at about 500 lp/mm". The resulting prints were examined under a microscope to determine the resolution limits of the papers. He doesn't specify in the article the exact standard used to visually judge extinction of the bar pattern. However, I doubt very much whether he's fallen into any methodological traps here - Ctein is a first-rate amateur scientist and understands well the very legitimate issues you raise, with respect to both experimental design in general and psychophysics in particular. And his conclusion is that paper resolutions are *far* beyond the point where it makes a difference visually, a finding that should be robust to reasonable variation in judgment thresholds among experienced observers.

  2. #12
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    Resolution limits of prints

    Thanks Oren, that's very helpful. It also confirms what I suspected, and is a near perfect demonstration of why resolution data, by itself, is useless ... unless you're buying a spy satelite.

    I believe the results of his test completely, and for reasons that I stated in my post earlier, believe that those results are essentially useless and have little bearing on real world photography.

    I'll go over the reasons again:

    1) The test target, like all resolution targets, is a hard-edged, 100% contrast subject. Very little of what you photograph in real life, unless you operate a process camera (if they exist anymore) is anywhere near 100% contrast. And few edges in nature are as abrupt as test target edges.

    2) The results were evaluated by microscope, and while he doesn't seem to mention how resolution was determined, I'm willing to bet that it was based on line pairs being distinguishable from each other in any way. Under ideal viewing conditions of a test target, this could mean that lines rendered at 1% or 2% contrast were considered rendered. In real life, however, you would not consider those lines to be rendered detail; you'd consider them to be hazy background noise. Under normal viewing conditions, fine detail is invisible below 5% contrast; it doesn't look like real detail until around 20% contrast; and it doesn't look sharp until around 60% contrast.

    If the MTF curves of the paper were to show a long slope, declining below 5% contrast after the resolution exceeded 150 lp/mm, it's likely that you would see no detail finer than 150lp/mm in a magnification of real printed negative--even if test target detail might still be detectable under ideal conditions at four times that resolution.

    3) This part I didn't guess--his use of a press to force the test target onto the paper may well reduce the most significant resolution limiting characteristic of the paper: its texture. If a physical press flattens the surface of the emulsion and guarantees near perfect contact, it would make sense that you'd get higher quality results than you might with more conventional (real world) techniques.

  3. #13
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    Resolution limits of prints

    Paul -

    Thanks for your response, I appreciate the prod to be more careful about defining the relevant issues.

    First, I agree, to really understand what's at stake here we should be talking about MTFs rather than resolution, which even when unambiguously defined with respect to contrast is only a single slice through the overall modulation transfer characteristic.

    So more precisely, the fundamental question is, are paper MTFs limiting factors with respect to any important perceptual characteristic (discernible spatial detail, degree of apparent sharpness, anything else?) of pictorial images made on the papers?

    At what spatial frequency does a paper's MTF decline below the (frequency-dependent) threshold of perceptual relevance? I speculate that Ctein's stated resolution figures, even if they do represent minimal contrast (which is a speculation on your part, and which is not certain) represent a point so far out on the spatial frequency scale that they could be consistent with MTFs that are indeed perfect with respect to anything that matters visually. But I take your point that Ctein's experimental results, at least as reported, do not prove this, and do not directly resolve (no pun intended <g >) the basic question.

  4. #14
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    Resolution limits of prints

    since MTF testing is beyond anyone's means at home, why not do an informal test with your own prints? Next time you have an oportunity to scan a print that you've made in different sizes (ideally one of which is a contact print) scan all of them at a very high resolution. Res them down in increments, and notice at what point you see detail becoming obscured.

    There's probably a better way to do this test; if you can think of one, let me know. Maybe it would make sense to do multiple scans and different resolutions.

    My observations came from scanning many prints, all different sizes, including contact prints. Many of them extremely sharp and detailed. I noticed that in all cases there was no difference in visible detail between 360 dpi and 720 dpi ... not counting paper surface detail. But large prints from the same negs revealed detail equal to many times this resolution. This was done on an epson 1640 XL (at work, sad to say).

    A friend of mine who does copy work as part of his job and who's been doing piezography since the beginning has had the same findings. He works with medium and large formats, including 5x7 (enlarged) and 8x10 (contact) so I trust he knows what detail looks like.

    Again, none of this is science, but it reflects a pretty systematic survey of real live photographs. Scientific experiments, which have to balance accuracy and repeatability with limited means, often end up designed in ways that fail to mimic the real world situations they're attempting to investigate. The resolution test in its classic form is one typical example.

  5. #15
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    Resolution limits of prints

    >> Scientific experiments, which have to balance accuracy and repeatability with limited means, often end up designed in ways that fail to mimic the real world situations they're attempting to investigate. <<

    This is true, alas.

    This interesting experiment will have to wait - I have no scanner at the moment, though I'm sure I'll end up with one eventually.

    I'm a little nervous about the possibility of artifacts arising from the particular algorithm used to "res down in increments" from a high resolution scan, and also about artifacts that may arise when sampling frequencies start to approach critical spatial frequencies in the image. Doing original scans at different resolutions, as you suggest, may shed some light on this, although one would still need to understand what the scanner itself is doing differently in hardware and/or software to achieve the different resolutions.

    >> the most significant resolution limiting characteristic of the paper: its texture <<

    This may be relevant to an observation I have made in my own experiments, that contact prints on glossy RC do seem to retain more information than comparable prints on glossy FB. I'm not sure whether this is a perceptual illusion or an actual difference in fidelity of reproduction under my contact printing setup, which provides pressure that's adequate for my purposes but not anywhere near what Ctein applied to assure optimal contact in his experiments. I do have a high-quality resolution test slide floating around somewhere; it might be interesting to dig it out and try contact printing it to see whether there's an actual loss of information in an FB compared to an RC print under my usual working methods.

  6. #16

    Resolution limits of prints

    Assuming that you are scanning your own work and that it does not involve 37 steps and weeks in the darkroom making prints, I wonder why you are not scanning your negatives? Removing the optical printing generation will make your job lots better. You could completely avoid the limitations you describe in scanning prints and end up with very high resolution files with more detail than you can put on paper in a book.

    What process will you be using to reproduce your photos for your book? The real task is to determine how much resolution you need to reproduce all the detail that your ink and paper will display and of course the size of each image in reproduction. Now if you're exercising your mind, wondering aloud or you are working with historical material and have only the prints, pardon me for butting in. Otherwise, scans from negatives and press proofs are in order to determine what works for your print job. You really do need to start with the desired end result in mind and work backwards from there. - which seems to be left out of the conversation until now.

    As to settling the issue of print resolution with strict testing and MTF graphs, I'm pretty sure that if you look at some photos you can pick the best example. Its pictures - and the one that looks best - is best.

    Finally, from the original post: "but unless I'm missing a huge variable, the paper is acting as strong resolution limiter for real world images under real world viewing." The paper is not limiting resolution for viewing by human eyes, but from lots of experience I can say that paper prints will limit resolution for further enlargement. And why wouldn't it? You're making a copy of a less than perfect copy.

  7. #17
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    Resolution limits of prints

    Hi Henry, thanks for your post. I definitely agree, especially with your conclusion that the eyes are (for all practical purposes) the limiting factor when it comes to fine detail resolution in prints.

    I wrote the original post not to describe a problem I'm having, but rather to let people know that there's probably no reason to scan prints at computer-choking resolutions (or to scan negs at a resolution that will be huge at their final print size).

    And also because I thought it was interesting.

    I'm actually going to do a combination of scanning negs and scanning prints for my project. I'll scan the negative that haven't been printed already, and also the negs of images that have too much important detail in deep shadow--the scanner doesn't seem to do a good job of seeing that far into shadows, even in 16 bit mode.

    Otherwise, I'll scan the prints, since the results are more than sharp enough, and because I've already done hard time making the prints and getting the values where I want them to be.

    The book is going to be a small edition artist book, printed ink jet with piezography. The book prints will be the exact size of the original prints. I may end up redoing scans if I'm not getting the results I'd hoped for. In some cases the scans from the prints may not translate well into the tonal scale of piezography, in which case I'll have to go back to the neg and rethink the whole print.

    I've been scanning at 400 dpi at final print size, which is a little higher than the highest resolution that seems to make a difference. I like to leave a little headroom to minimize the effects of any future manipulations. All the scans are 16 bit.

    Scanning the negs has so far been easy, but I've only made fpo scans at a relatively low resolution. I'm sensing that I may not know what I'm doing as far as maing the best final scans from the negs.. If there are any tricks to getting excellent scans from bw negs (in terms of getting the tonal scale accurate, not in terms of resoultion) ... or any good books I should read, I'm open to suggestions.

  8. #18

    Resolution limits of prints

    Paul, You are taking on a job that used to require several highly skilled people and some expensive equipment! At least you don't have to worry about color separations since its B&W.

    Its been a year since I've made any Piezo prints. My printer head clogged for the last time and I said "to hell with it" and have not touched it since. When it worked, I did make nice prints, some in a book of prints documenting the construction of a library that was well received. (about 27 images, edition of two, one for the donor and one for the institution) I found with my printer set up (the old Piezo plug-in for Photoshop and a 1200 with Piezo inks) that more resolution up to about 720 PPI at repro size seemed to show in the prints.

    When I tried to scan darkroom prints I found that intricate detail got lost unless I printed about half size of the original - then they looked pretty nice but still not like a good scan of a negative. The reason I tried this was to see what would happen when attempting this workflow, knowing that it was likely not a good solution. I was right about that part! I've scanned many prints, negatives and produced thousands and thousands of digital captures for offset reproduction and done some historic restoration of old prints so I had a good idea of what to expect. I was fascinated at the time with the observation that inkjet printers could often make a very good looking print from what I considered to be poor quality files from digital cameras. You have probably observed this - a file with all kinds of artifacts, jaggies, softness and flaws that makes a good looking Epson print.

    If you are working with negatives that printed easily in the darkroom, they will probably scan nicely as well. I don't think any book will help that much with scanning. (there may be one but I don't know of it) Not nearly so much as just doing it and trying different settings and adjustments. I usually scan in 16 bit after setting black and white points and bring the file into Photoshop to do most adjustments starting with global levels and curves then spotting and going on from there. Depending on your scanner and its software this approach may work well for you - only course adjustments in the driver then manipulate as desired in Photoshop at 16 bit. A very nice side benefit of this method is that you free yourself from the vagaries of different scanner software and bring your workflow into one unified stream. This helps a lot if you use more than one scanner or you have one that is supplied with crappy software.

    Good luck on your project!

  9. #19
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    Resolution limits of prints

    Thanks Henry, you're right that I have my work cut out for me. Soon enough I'll get to compare scans from prints with scans from negs ... I'll post the results. In the mean time I have other worries, like editing and sequencing ten years of work, learning bookbinding, and getting someone to pay for all this!

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