[sane-devel] Affordable film scanner

[Major A]

> The Canon FS27XX scanners are cases where you need a large gamma
> to obtain good results (2.0-2.5). Right now the backend collects
> 12-bit data and converts them to 8 bit with a 4096 byte gamma
> table (or exports them as 16-bit data). This usually works well;
> but I admit that there are situations where a few more bits of depth
> would be welcome.

That's the case for Coolscans just as well.

> Even if your scanner has got exposure controls, the question remains
> whether it scans with fixed settings and manipulates the raw
> digital data, or whether it really adjusts the gain and zero of
> the amplifiers. This is probably the point where some companies
> try to save money; good analog circuitry is expensive.

Analog gain doesn't do too much good because the limiting factor is
always the CCD, not the ADC -- you would only be amplifying extra
noise if you increased analog gain. What Coolscans do is to let you
allow to change the exposure time for each channel separately. This
way you get the best of it all -- twice the exposure means sqrt(2)
better signal-to-noise ratio.

Changing the exposure separately per channel is a bit tricky with
scanners that use a white light source, since all colours are scanned
at the same time, whereas the Coolscans scan one after the other. But
it should still be possible to change the overall exposure time -- are
you definitely certain that the FS27xx doesn't allow you to do that,
or is it just the current drivers (Window$ or SANE) that can't do it?

  Andras

===========================================================================
Major Andras
    e-mail: andras@users.sourceforge.net
    www:    http://andras.webhop.org/
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