Carla, in addition to what’s been posted already, there’s at least
one more thing to be aware of. The latest scanners have a combined
light source and sensor assembly, which no doubt has a fancy name,
though I don’t know it. The main advantages of this are tighter
control of the relative locations of the parts, easier
manufacturing, fewer parts and so on. There are two practical
outcomes for the user. One, the scanner can be made much thinner
(and be more readily incorporated into all-in-one machines) and,
two, the depth of field is very much reduced.
This last means that paper in contact with the platten is focussed
properly, but anything raised slightly will not be in focus. If
lack of focus rather than resolution is your problem then this may
be the cause. Nothing much you can do about it in that case, except
make flat jewellery!
Kevin (NW England, UK)