Not sure if this is a FAQ. I scanned some roll film with the holder and top sensor. Then I tried to scan the film without holder. The top cover was removed to expose the top sensor.

But with the top cover removed, the film is not flat, but curved. So I just wonder if I should scan sheet film (or any other film without holder) the same way as positive print and use the bottom sensor instead. Then the top cover would be in place to push down the film/print to the flat.....