Digital BW, The Print

----- Original Message ----- 
From: "Austin Franklin" <darkroom@...>
To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
Sent: Tuesday, May 27, 2003 10:40 AM
Subject: RE: [Digital BW] Re: Scanning


> Truman,
>
> My experience through literally thousands of hours of design and testing
> digital imaging systems is that all you do when you increase the exposure
> time is simply shift the data values, it does not increase the dynamic
range
> unless the system has a limitation in the first place.  If this limit
> exists, what you CAN do is take multiple exposures with differing exposure
> times.  This WILL increase your dynamic range, if done properly...but, the
> limitation, as always, is going to be noise.
>
> In a correctly designed system, the data out of the A/D is limited by the
> noise floor of the CCD, as the A/D is matched to the output noise of the
> CCD.  This means that expanding the voltage into the A/D doesn't buy you
> anything, as you will only be converting noise in the lower bits.
>
> Point is, as a general rule, you can't say that increasing exposure time
> increases dynamic range, it is very design dependant, and as you probably
> know, dynamic range is limited by noise if the overall signal stays the
> same.
>
Austin,

Of the CCD scanners generally in use here, flatbed and film, does the user
have any actual control over the physical operation of the scanner or is it
all just software manipulation of the data after the analog to digital
converter?

I know some scanners allow you do things like adjust focus, run multiple
passes to reduce noise or control the speed of the scanning head to adjust
resolution but this would not seem to have little or no effect on the
dynamic range of what you ultimately get in your scan file.

It seems to me that the scanning software presents "controls" in a manner
that gives the impression the user is actually effecting the function of the
scanning operation itself when in reality they are not. Hence a great deal
of confusion among the users as to what they are actually accomplishing when
the tweak the scanner controls.

Martin

(snip earlier)