I have found that one or two sheet-fed scanners do address this situation by having two settings, one for these objects and another for thicker objects such as photographs or cardboard or business cards that allow them to pass straight through without curling around the scanner head. While this approach works fairly well it is still not up to par when c omparing the results with flatbed scanners. If you look close at a sheet-fed scan on thick objects you will typically see some jagged edges. Flatbeds don't have to deal with paper stretching or rollers that wear out so they are generally a bit more prec ise in scanning the object. At the low end it is a matter of trade-off.
Flatbeds typically cost a bit more and take up considerably more room.
On the plus side flatbed scanners, even those on the low end of the totem pole, offer at least 300x600 dpi while this is not always the cas e with sheet-feds. Flatbeds can also scan just about anything you can place on the bed, books, magazines, photos, or the family dog. When you pick a scanner and have decided between a sheet-fed or flatbed the next thing is to look carefully at the specs, particularly the resolution. At the core, scanners work by turning your typed pages, photos, and drawings into rows of tightly packe d dots. These dots are stored as bits in a file called a bitmapped image file.