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Thread: Geometrical distortions on belt drive scanners

  1. #1

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    Geometrical distortions on belt drive scanners

    Hi! Finally I want to expose some findings: scanners where the scanning carriage moves with the belt - distort. Scanners where the process is made by lead screw - do not distort. Sad, but true.

    I found it accidentally while comparing different scans of the same 35mm negative. I will not show mine, but let's look what he have on the net.



    The moving gif is made from two crops from that page:
    http://trichromie.free.fr/trichromie.../03/02/ScanC41
    Two scans were made on IQSmart3. On a gif you can see the difference - it moves. What can be if the face will be scanned in that area? I tell you: nothing good. The face will loose right emotions and will be distorted.

    If one wants to explore that, he can visit the page:
    http://www.largeformatphotography.info/scan-comparison/

    There is a lot to compare. And nearly only drum scanners do not distort. They simply have lead screws. Do not distort: Topaz, Nexscan and Smart 3XX. They also own lead screws.

    Having said all that on a large format photography forum I understand that the larger the original the less is the distortion effect. Anyway I think it is good to know that such thing as a geometrical distortion of the belts exists in this not perfect world.

    Take care
    Al
    Attached Thumbnails Attached Thumbnails IQsmart3_2.gif  
    Last edited by SURF; 20-Nov-2017 at 15:32.

  2. #2

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    Re: Geometrical distortions on belt drive scanners

    Surf,

    Now that you got my attention, I will state that you are correct in the sense that after a while belts do tend to stretch and they should be replaced every so often. Yes, the Scitex 340 series do have a lead screw that will require just a wipe with a a bit of dustless grease (sorry, forgot the actual name as it has been a long time!)

    As for the Imacon, Hasselblad as part of their servicing, they DO REPLACE the belt on their scanner. Therefore the "chattling" by the lead screw will not "pop" out at you.
    As you mentioned in the previous thread, that noise is from a stretched belt that was probably never serviced. I asked about replacing the belt on an Eversmart Pro II scanner, but never received a direct answer. Not sure it was an easy thing to do, but I would definitely like to have it done on my Eversmart Pro II. Can anyone on this forum who had repaced their belt, tell me if was an easy thing to do or was it more technical in nature making it harder? (CORRECT TENSION & BELT POSITION?)

    Thank you for bringing this to my attention, I do appreciate.

    Quote Originally Posted by SURF View Post
    Hi! Finally I want to expose some findings: scanners where the scanning carriage moves with the belt - distort. Scanners where the process is made by lead screw - do not distort. Sad, but true.

    I found it accidentally while comparing different scans of the same 35mm negative. I will not show mine, but let's look what he have on the net.



    The moving gif is made from two crops from that page:
    http://trichromie.free.fr/trichromie.../03/02/ScanC41
    Two scans were made on IQSmart3. On a gif you can see the difference - it moves. What can be if the face will be scanned in that area? I tell you: nothing good. The face will loose right emotions and will be distorted.

    If one wants to explore that, he can visit the page:
    http://www.largeformatphotography.info/scan-comparison/

    There is a lot to compare. And nearly only drum scanners do not distort. They simply have lead screws. Do not distort: Topaz, Nexscan and Smart 3XX. They also own lead screws.

    Having said all that on a large format photography forum I understand that the larger the original the less is the distortion effect. Anyway I think it is good to know that such thing as a geometrical distortion of the belts exists in this not perfect world.

    Take care
    Al

  3. #3

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    Re: Geometrical distortions on belt drive scanners

    Thank you for the interest Evan.
    Quote Originally Posted by Evanjoe610 View Post
    Can anyone on this forum who had repaced their belt, tell me if was an easy thing to do or was it more technical in nature making it harder? (CORRECT TENSION & BELT POSITION?)
    The first time it is not too easy.

    Quote Originally Posted by Evanjoe610 View Post
    As for the Imacon, Hasselblad as part of their servicing, they DO REPLACE the belt on their scanner. Therefore the "chattling" by the lead screw will not "pop" out at you.
    As you mentioned in the previous thread, that noise is from a stretched belt that was probably never serviced.
    First thing to start is measuring. You have to learn how to measure the distortion. On belt driven scanners it is always visible, I think due to the nature. You also have to decide when to stop and when to start worrying about it. I must say that it is easier to live without knowing about it. Not alike lead screw the belt can wear differently at different positions on the table and after all there is at least one more critical part inside the scanner: it is reduction gear.

    As for the source of new belts. I don't think Scitex made special belts and I believe that best quality industrial belts can be used. I have examined the site of Gates and found the belts that look alike in any aspect.

    I think it is a good idea to PM me.

    Regards
    Al

  4. #4

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    Re: Geometrical distortions on belt drive scanners

    Al,

    Thank you for your advise. I have PM you to carry our conversation off line.

  5. #5

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    Re: Geometrical distortions on belt drive scanners

    I don't think there are issues with distortion on the Eversmart or IQsmart line of scanners. They were made in part for copydot work and distortion would really mess things up there.

    One thing that you may see is a difference from an XY stitch machine like the Eversmart and a xy zoom (stitch free) machine like the Aztek plateau. The stitched scan should be more accurate due to the final scan being made by multiple "telephoto" shots while the plateau uses one scan. You might see natural differences due to the perspective of the lens in the scanner.

    Creo Scitex scanners go through a long calibration process where geometric distortion and alignment is checked.

    As far as belts, for the creo scanners there is newer and older belt. The newer one is a little better and it is the white belt.

    Which belt driven scanners have you seen distortion in? I could imagine it in a cheap one... But a Creo Scitex?

  6. #6

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    Re: Geometrical distortions on belt drive scanners

    Quote Originally Posted by 8x10 user View Post
    I don't think there are issues with distortion on the Eversmart or IQsmart line of scanners. They were made in part for copydot work and distortion would really mess things up there.
    Problems with distortions do not influence stitch or copydot process.

    Quote Originally Posted by 8x10 user View Post
    Creo Scitex scanners go through a long calibration process where geometric distortion and alignment is checked.
    We are talking about tens of microns here and it has no influence on calibration too.
    Quote Originally Posted by 8x10 user View Post
    As far as belts, for the creo scanners there is newer and older belt. The newer one is a little better and it is the white belt.

    Which belt driven scanners have you seen distortion in? I could imagine it in a cheap one... But a Creo Scitex?
    All I know: the newer - the cheaper everything goes. Kinda joke.

    Yes. Creo Scitex. The gif was made from IQSmart3 scans and I see no big difference with that also on Eversmarts, Screens and Imacons.

  7. #7

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    Re: Geometrical distortions on belt drive scanners

    OK. Time to make homework.

    Scan the original at 3200 dpi. Scan area: 10mm by 10mm. Then rotate original 90 degrees on the table and repeat scan. Save both at different names.

    If you were precise in 90 degrees you can only crop the area. Start from upper left corner and crop it close to one pixel accuracy. Then crop the bottom right corner. See the sizes in pixels and scale one scan to have both of exact size. Then you can compare the two by fast switch. What moves - is the distortion. The step of it in Eversmarts is 5 mm. You can also try to measure the absolute size of the distortion. Average old scanner has about 3 pixels. Nothing good, nothing too bad. The scanner from the first post was measured like 4 pixels. No good.

  8. #8

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    Re: Geometrical distortions on belt drive scanners

    How can you be certain that your image was perfectly alligned along on the glass in the first place and how one could be certain about rotating it by 90 degrees precisely ?

    What about the calibration ? Was it recently done?

    Then for Eversmarts there are certain factors such as Head Tilt and Rotation. When the last time were they checked and adjusted on the scanners produced the images for analysis?

  9. #9

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    Re: Geometrical distortions on belt drive scanners

    Quote Originally Posted by SergeyT View Post
    How can you be certain that your image was perfectly alligned along on the glass in the first place and how one could be certain about rotating it by 90 degrees precisely ?
    I will answer only one question. You have to rotate one image back 90 degrees in fotoshop afterwards. I have fogotten to mention it. It's a good idea to have some lines on the original, so you can fine tune rotate in ps. In other words it is absolutely not necessary to be precise in the original placements. You can fine tune it in the program.

    Other questions have no connection with the problem.

  10. #10

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    Re: Geometrical distortions on belt drive scanners

    Now I want to post scanner comparison based on the pictures from well known page:

    http://www.largeformatphotography.info/scan-comparison/

    Have a look how Eversmart Supreme distorts geometrically compared to Tango drum scanner. And what is interesting: all Tangos do not have any distortion when compared to each other. There are several Tangos and two Eversmart Supremes. Both Supremes heavily distort.



    Supremes are often used in the museums for backing up the images. I believe that most of that backups are good only for trash folder cause they are distorted. What do you think?
    Attached Thumbnails Attached Thumbnails ESSupreme_vs_Tango.gif  
    Last edited by SURF; 20-Nov-2017 at 15:44.

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