RE: filmscanners: Goals in adjusting scans?

["Austin Franklin" <darkroom@ix.netcom.com>]

> With 8bit, you don't have much scope for adjustments after
> acquisition, so
> should get as close as possible to the final, desired result. The
> reason is
> that with 8 bits, you get rounding errors during successive mathematical
> operations on the data.

That's kind of only part of the reason.  It has to do with re-mapping 8 bit
values to 8 bit values, you lose codes ('codes' meaning the individual
values from 0-255).  when you map the values of, say, 0-15 to 0-20, you are
going to get 'gaps' in the codes, since you are mapping 16 values into a
range of 21 values, the result is 5 codes that don't have any values.  This
is what causes 'combing' on the histograms...which is missing codes.

Anytime you 'move' the data, ie, re-map, you are going to have code values
that have no data (mapping smaller number of codes into a larger code
range), and multiple data values that are combined into a smaller number of
codes (map a larger number of codes into a smaller range).

This is why it is very important to do your adjustments in 'high bit' (which
is what you do when you make adjustments in the scanner software, or scan in
high bit mode and send that to PS)...where you don't have this limitation.