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Thread: Aliasing and scanning resolutions

  1. #21

    Aliasing and scanning resolutions

    Chris,

    I think we're saying somewhat the same thing. As to the bicubic sharper, that one is hotly debated. I personally use regular bicubic for most downsampling. Then again, as most of my workflow is digitally based, I rarely downsample....I just print at the native resolution.

    I agree, most of the problems associated with scanning could be reduced or solved by using a higher quality scanner. But as most are using the Epsons or Microteks, downsampling does provide a valid reduction in aliasing effects. I notice this far less though on my Imacon than friends with Epsons! I rarely downsample at all....even with film scans.

    By the way Chris, I won that auction for your book that was mentioned in these threads on Ebay. I should be getting it any day now.

    All the best.

  2. #22

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    Aliasing and scanning resolutions

    Various interesting points have been raised in this discussion. Let me ask one other related question.

    Think of each step in the process as a filter which affects the spatial frequency response of the image. That frequency response is described by an MTF curve giving percentage reduction in response as a function of frequency in line pairs per mm. Often that is summarized by a single number which presumably gives the highest usable frequency, but that is an over simplification. Sampling theory says that you can't resolve a frequency higher than twice the sampling frequency, for digital sampling. Aliasing in a related but different issue. It concerns what happens to higher frequencis, if they are not filtered out. They are reflected or aliased down as lower frequency artifacts.

    The last step in this process is printing. If you print at 240 ppi, i.e., 9.45 pixels per mm (ppm), your maximum detected frequency would be half of that or 4.72 lp/mm. If you print at 360 ppi, that maximum increases to about 7 lp/mm.

    Similar reasoning applies to scanning (and indeed all steps in the photographic process). The Epson 4990 can scan at 4800 ppi, or about 189 ppm, and that translates to about 94 lp/mm. In fact, most reports suggest it does not do nearly as well as that, with figures like 30-35 lp/mm appearing to be more realistic. Let's take 32 lp/mm for the sake of argument. Enlarging 4 X reducing that to 8 lp/mm. (However, that figure may represent something somewhat down from the top of the MTF range, which is what the Nyquist limit presumably gives you.)

    Now there is no simple way to combine successive filters. Ideally you would multiply, point by point, the MTF values, but since those curves are seldom available, other rough rules of thumb are used. The important thing to note, is that you are always going to get resolution in lp/mm lower than that which the coursest filter provides.

    This reasoning would suggest that even printing at 360 lp/mm is not going to yield much above 5 lp/mm, which is the commonly used estimate of what the human eye can detect at 10-12 inches from the print. It also suggests that the print resolution is the most limiting factor in the entire process.

    Any comments?

  3. #23

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    Aliasing and scanning resolutions

    To me one of the upshots of Leonard's analysis is that the Epson 4990, when set to its maximum, appears to deliver somewhere in the neighborhood 1/3 of its promised resolution. Other scanners do a better job, but very few consumer or prosumer scanners deliver as advertised. The files they create may contain lots of data - but much of it is...junk.

    So if I were Sandy, I would presume that the 1600 ppi setting is likely to deliver much closer to 1000 ppi at best, and that an enlargement of 3x would be the practical limit, if we want to retain critical sharpness.

    That being said, one of Sandy's images in a 36x60 print, critically sharp etc. would be quite something to see.

  4. #24

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    Aliasing and scanning resolutions

    > This reasoning would suggest that even printing at 360 lp/mm is not going to yield much above 5 lp/mm, which is the commonly used estimate of what the human eye can detect at 10-12 inches from the print. It also suggests that the print resolution is the most limiting factor in the entire process.

    Even photopaper is limiting in a lot of situations. One more factor is sharpening. By increasing the edge contrast, you increase the eye's ability to resolve the fine detail and thus the perceived sharpness. That is why some folks have noticed that a scanned and processed image can be sharper than a contact print of the same same image at the same size. Sharpening changes a lot of the issues for how sharp a print looks and is a key component of scanning. It does not create new data, but scans have more data in them than appears to the eye - sharpening restores this.

    > So if I were Sandy, I would presume that the 1600 ppi setting is likely to deliver much closer to 1000 ppi at best, and that an enlargement of 3x would be the practical limit, if we want to retain critical sharpness.

    The better microteks get to their stated resolution - Paul B found that his 1800 really did do 1800.

  5. #25

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    Aliasing and scanning resolutions

    Okay, so what's the bottom line here? I'll continue to print at the enlarger (or will when I replace the broken lens) because it looks like regardless of inkjet or enlarger we end up with the same so-called "resolution" and I have infinite degrees of gradation within the tones of concern, rather than a troublesome maze of steps and complications leading, hopefully, to the same outcome with scanning and printing. No? My eyes tell me Yes.

  6. #26

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    Aliasing and scanning resolutions

    Bottom line? Digital is different from analog silver. Scanning can give you sharper prints than you can get with an enlarger, but they will not be silver prints. If you want silver, and have the resources to use it, quit reading these digital threads and stop worrying about it.:-) If time and space were not an issue, I would print with silver myself. Since they are, I waste time trying to get the best results from what I use, just as I did when I used a darkroom.

  7. #27

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    Aliasing and scanning resolutions

    Scanning can give you sharper prints than you can get with an enlarger,

    Not according to the posts I consider most credible. Seems the outcomes bottom out to within 8lp/mm.

    ... still working on my home-made pin register for unsharp masks.

  8. #28

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    Aliasing and scanning resolutions

    "The better microteks get to their stated resolution - Paul B found that his 1800 really did do 1800."

    What standard targets and/or procedures are people using to test this?
    For discussion and information about carbon transfer please visit the carbon group at groups.io
    [url]https://groups.io/g/carbon

  9. #29

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    Aliasing and scanning resolutions

    Air Force standard on glass:

    http://www.butzi.net/articles/scannersoft.htm

    These sell for about $130. I wish the guys doing the scanner tests had one. They cut through the speculation pretty quickly.:-)

  10. #30

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    Aliasing and scanning resolutions

    www.butzi.net/articles/scannersoft.htm

    Impressive. I actually have silverfast software and never used it. (Thanks for that, page Paul!)

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