Preferred scan orientation for all-in-one scanner

Question

I want to scan a photograph using the flatbed scanner in my all-in one printer (Samsung C460W). The long side of the photograph is shorter than the short side of the scan bed, so I could scan it in either landscape or portrait orientation.

Would either of these orientations produce a higher-quality result? The specifications say it can scan at 600x600 dpi, but I assume that in the scanning direction this is achieved by the movement of the scan had, whilst in the perpendicular direction this is limited by the scan sensor's resolution. So if I align e.g. the short size of the photograph with the short size of the scan bed, I am not utilizing the maximum possible sensor photocells.

Does either of the two directions have a higher quality and if so, which one?

(I cannot see a striking difference in images scanned in the two different directions, so I suppose the differences - if any - are small. But I am still interested to know whether there is an academic difference.)

Answer 1

As you've already noticed, there is little practical difference between scans taken at different orientations in most cases.

• Scanning images at an angle may help reduce moiré.

• Placing items on one side or the other of the sensor will slightly change the perspective of the scan on scanners that use a lens to focus onto a linear sensor. This can be used to scan different images contained in lenticular prints, so the 3D effect can be recreated.

  • Your scanner uses a Contact Image Sensor (CIS).

• Some scanners can move the scan head in finer increments than the resolution of the sensor. Scanner specs will have different values for x and y. For example, 600x1200. In this case, there is a potential improvement if more of the image is orthogonal to the sensor when scanning at resolutions higher than the sensor resolution.

  • Most prints do not benefit from scanning at resolutions higher than 200-300 dpi.

  • Your scanner has a resolution of 600x600.

• If there is a defect that affects the sensor or motor, orienting the image orthogonal to the defect may reduce its effect on the image.

  • Fixing the defect or using a different scanner would result in much greater improvements.

• The scanner motor movement may not exactly match the sensor resolution. The dimensions of scans may be slightly different when the source material is oriented differently.

  • The difference is usually only a few pixels over the entire document.

  • A related issue occurs when pages slip on the rollers of form-feed scanners.

Answer 2

For a flatbed scanner, the most important difference orientation will make is the time it takes to make a scan. The shorter the distance the sensor array has to move the faster the scan will be.

Practically, rather than theoretically, speaking the faster the scan the more likely the operator is to correct operator errors by virtue of there being more time to do so. Practically speaking, operator error is by far the most likely source of low quality scans...all inputs being equal of course.

Mechanically, 600 dpi is 0.042mm. About a hair's breadth or 40μm. A working precision for an ordinary stepper motor turned rolled ballscrew is about 20μm. 40μm is approximately the Nyquist limit (2x the sampling rate) of the mechanical action to avoid moire.

On the one hand, such precision is virtually magic compared to a typewriter or a lawn mower engine. On the other hand, it is commodity engineering using cheap off the shelf parts and ubiquitous manufacturing processes.

The mechanical delta between scan orientations is less than rounding error compared to the delta between human processing decisions and software packages.