How can I find a modern flatbed scanner with a reasonably large depth of field? Here "reasonably large" would be something like at least 2mm of DoF above the scanner glass.
Typically DoF is not listed anywhere in the technical specification, and few people are interested in testing it when they are reviewing a scanner.
Based on what I have learned so far, it seems that older scanners often had a fairly deep DoF. For instance, my old (> 10 years) HP ScanJet 4200C performs reasonably well.
However, many modern scanners seem to have an extremely shallow DoF. My guess is that this is related to the fact that older scanners used a CCFL lamp while a modern scanner often uses LEDs. With LEDs, the scanner can be made much thinner. Perhaps the small physical size of the scanner favours a lens that has a shallow DoF?
But this is just a guess, as I don't really know anything about the construction of the "lens" that is used in scanners. Can one somehow explain it in "SLR terms"? Does it make sense to use terms like focal length and aperture in the context of a scanner?
Some background: I am abusing my scanner as a digital camera; I use it to take pictures of nearly flat objects. For example, if you need a quick snapshot of a handmade greeting card (flat decorations glued to a cardboard background, etc.), a scanner is surprisingly convenient in comparison with a DSLR: you don't need to worry about aligning the camera perpendicular to the object or setting up lighting, and you won't have any lens distortion that you need to fix in post.
However, this is possible only if the scanner has enough depth of field above the glass.