I scanned a postage stamp.

Here's the scanned image, as displayed in VueScan, before selecting a crop area:

enter image description here

After selecting a crop area, the same image looks like this:

enter image description here

I assume that VueScan is using the average luminance/colour values to adjust its assessment of the values it has scanned, but:

  • what exactly is it doing?
  • why is it doing it?
  • is it possible to prevent it, so that it displays only scanned, and not adjusted, values?

If any of the scanner settings are set to "Auto", only the selected area will affect the calculated result. When you crop out most of the very bright areas of the entire scan, the remaining areas will be brightened more to give the entire selected area the same "average" brightness that the entire scan area had before you cropped it.

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