There is a second issue related to "movement during multisampling (or multiscans for that matter) and that is the accuracy of the stepping kotors in the 4990 and the similar machines from Microtek and Canon. At the $500 price point they just do not ahve the engineering for the rigid percision to keep the scanning array 100% perfectly aligned from pass-to-pass. Good enough that it works some of the time or even most of the time, sure since we are talking about very small measurements but all of the time, no. In fact, one of the things you pay a lot more money for in the higher quality scanners is the added precision of the stepping motors achieved through both mechanical and firmware differences with the lower priced units. It's somewhat like we used to say in the television broadcasting industry 35 years ago .... a quad (broadcast video) recorder is $50,000 - to $100,000 worth of recording equipment with an additional $300,000 to $400,000 of error correction mechanics and electronics attached. A lot of that applies to scanners as well.
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