Why can a flatbed scanner stretch an image differently depending on scan orientation?
Asked 5/20/2020
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I scanned the same printed image on a flatbed scanner in both portrait and landscape orientation, then rotated one scan 90° in GIMP to compare them. When overlaid, one scan appears slightly stretched: if I align one side, the opposite side drifts out of alignment. I saw the same effect on two consumer scanners.
Why does this happen on flatbed scanners? Is it caused by the sensor/light angle, scan resolution settings, or the scanner’s transport mechanism? Are there scanners that avoid this kind of geometric distortion?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
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Why does this happen?
A flatbed scanner works with two different processes, on each dimension.
One with a rigid element, the CCD array, which has a fixed dimension.
But the other dimension is given by movable parts, so this movable mechanism is not that precise on your case. It is normally by a motor pulling a belt.
Is it dependent on the angle that light is reaching the photo-sensors?
No, it is more about the motor-belt mechanism.
Are there any scanners that don't do this?
They are not common anymore, the rotative laser scanners, where you mounted your target on a cylinder, and each rotation was better to control the advance of the point where the laser targeted.
But you needed to scan all pice at one pass because what was imprecise was the mounting of the target.
A photo scanner is not precise also because you can put your target photo at a slightly different angle.
The best option is to scan on one "pass", and not dividing your project into different passes.
Originally by user37321. Source · Licensed CC BY-SA 4.0
user37321
6y ago
0
Generated from our catalog & community — verify before relying on it.
Yes—this can happen on flatbed scanners, especially consumer models.
A flatbed scanner measures the two axes differently: one axis comes from the fixed sensor array, while the other is created by the moving scan head driven by a motor/belt mechanism. Small inaccuracies in that moving system can cause slight stretching or compression in the scan direction. When you rotate and overlay two scans made in different orientations, those errors show up as mismatched geometry.
It’s generally not due to the angle of light hitting the sensors. It can also be made worse by software or scan modes that use non-square pixel resolutions, so it’s worth checking that you are scanning in a normal image mode rather than a fax-oriented mode.
In short: this is mostly a precision limitation of consumer flatbeds. Higher-end scanners tend to control geometry better, but some distortion is common in inexpensive devices.
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