[filmscanners] RE: scanner dmax discussion

["Austin Franklin" <austin@darkroom.com>]

Karl,

> So in part, the 'wizardry' of the electronics inside the scanner A/D
> converters is to try and compensate for the non-linearities of
> the detectors
> themselves.

That has nothing to do with the A/D converter, it simply divides it's input
voltage range by N steps, and records 1 bit per step.  The CCD sensors used
in scanners are relatively linear, but there is a calibration process that
does give a "table" of values, one per sensor element, that is applied to
the DIGITAL data after the A/D process.

> Note also, most of the detectors have photo response curves not that
> dissimilar from film, so by changing their detection threshold, you can
> drive them into different parts of the response curve.  Not all scanners
> allow you to do this, but in theory it is possible.

Actually, I haven't heard of any that do it that way.  All you have to do is
change the exposure time to do this.  I do believe that one claims to have
"analog gain", but that is not really analog gain as far as any electronics
between the CCD and the A/D are concerned, I believe it changes either
intensity of the illumination, or simply exposure time...I don't remember
which.

> There are scanners
> that allow
> you to scale (or autoscale) the image range to maximize the data
> extracted -
> but they are pretty pricey.

What scanners do you believe do that?  Typically all they do is apply
setpoints and expand the data so it occupies the full range of 8 or 16 bits,
but this is simply a conversion process, not an analog offset/gain function.
That COULD be done as an analog function, I've done it, but not on
commercial scanners, and there really is no need to do it anyway, as all you
are doing is expanding noise, so you really get no additional usable
information by doing it in the analog domain.

Austin

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