Ben,
I'm sorry if I made it too complicated. In effect, 'dpi' for a scanner comes down the same thing as 'spi' or 'ppi' . Any distinctions are too subtle to be concerned about in practice. If you want to call it 'dpi', do so. The reason for making a distinction is that 'dpi' is also used in descrbing printer resolution, where it means something very different.
An interesting question, which others might comment on is just what happens when you ask your scanner to return pixels at less than the specified optical resolution. Since you end up with fewer pixels in each line than there are sensors, there has to be some averaging done. Just where is this done: the scanner firmware/hardware or the scanning software? My conjecture is that the hardware will return half multiples of the optical resolution. So a 4800 spi scanner can return 4800, 2400, 1200, etc pixels per inch, and to return something intermediate, the software writer, e.g. Silverfast, has to do some further programming. I can pretty much see what the software would have to do, but I am not clear what the hardware/firmware combination in the scanner would do. For example, apparently the Epson V700 has a multiple lens system and does different things if you scan at 6400 than if you scan at 3200.
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