[filmscanners] comparing raw positive and raw negative scans

["Joe Pedit" <pedit@email.unc.edu>]

I recently scanned a black and white negative containing an image of a
MacBeth color card.  I scanned the film as a positive and negative with
a Sprintscan 4000 and saved the uncorrected raw scans in 12 bit mode.  I
then compared average pixel values for the different squares of the
color chart for each scan.  The results are at:
http://www.unc.edu/~pedit/chart.gif
The best fit line is:
negative pixel value = 256 - 1.25 * positive pixel value
Why is the slope -1.25 instead of -1?  Shouldn't the slope be -1 if the
scanner is truly outputting "raw" results?

Thanks,
Joe

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