Well, that is a good question.tsttm wrote: should i use a different mode?
If you really trust your lights to be uniform, you can go in manual mode, do some tests and remember the settings (aperture and shutter time) that better suit you.
That's exactly what you were doing, you just forgot to adjust shutter time after adjustng aperture, and the rule here is quite simple: if you reduce aperture by X stops you need to increase shutter time by the same X stops.
Otherwise, I would suggest to work in AP mode (aperture priority).
Set aperture to your desired value and take a test shoot.
The image will probably be under-exposed, with greyish paper, as the camera does not expect to have all that white.
To correct this, do NOT go in manual mode to change shutter time, but rather change the EV settings (the ones you get with the +/- button) until you get the "white" paper you prefer
With that setting, you basically are saying to the camera "calculate your exposure as you usually do, but after over-expose by X stops"
With this settings changes in other settings (fluctuations in light, zoom level, paper whiteness) should be automatically compensated.
But this is just my two cents, I haven't yet built my scanner so I am just throwing ideas.
BTW I think we should need to discuss better the photographic aspects of scanning, i.e. the "best" settings for each application & hardware...
For example: what about ISO???? I would say the lower, the better, without doubt, as long as you don't go in very long exposures, after all the cameras are well mounted and motion blur shouldn't be a problem.