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Micha, It's possible that the problem is the film itself, not the scanner. What kind of film did you use? It could be grainy. Check my page on grain,

http://www.normankoren.com/Tutorials/MTF8.html

The only size that matters is the pixel size, not the "image size," which is just the contents of a tag-- it can be changed without changing the file data. Be sure to scan at 4000 dpi. If you see grain sharply, the scanner is probably doing its job; maybe the camera lens didn't do a good job. That's my guess.