[filmscanners] RE: Repeated "Tonal correction", is it god?

["LAURIE SOLOMON" <laurie@advancenet.net>]

Austin,

I am not sure that I was really saying anthing different than you has said,
except I was neither as detailed or as articulate.  Your reply focuses on
the purely technical engineering aspects which may say that to do tonal
correction in the scanner is ok or even preferrable under given conditions;
but I was also looking at the practical reasons as well - one of which may
be that the scanner software does not allow for on the fly previewing of the
impact of a tonal adjustment prior to the scan and the tools may not be as
good as those in image editing applications.

I think the same is the case for the advisablility of doing secondary tonal
adjustments.  Technically it might be theoretically possible to do it with
16 bit files without any serious consequences; but practically speaking if
the adjustments are drastic (as both you and I caution against) or if the
later secondary and tertiary corrections  or adjustment involve expansion of
previously compressed - by previous corrections - parts of the tonal range,
then one might get some unanticipated consequential artifacts, noise, and
unwanted results.  This however, would be less likely if the secondary
adjustments were minor or involved  additional compression of the tonal
range.

At least that is how I am viewing it until proven otherwise.  But I do agree
with you technical and theoretical reasoning based soley on the technical
engineering aspects.  If you disagree with my reasoning, let me know; it is
the only way I learn is through argumentation.

-----Original Message-----
From: filmscanners_owner@halftone.co.uk
[mailto:filmscanners_owner@halftone.co.uk]On Behalf Of Austin Franklin
Sent: Thursday, March 13, 2003 9:09 PM
To: laurie@advancenet.net
Subject: [filmscanners] RE: Repeated "Tonal correction", is it god?


Hi Laurie,

> I would say first that I would be
> reluctant to do my
> tonal correction or adjustment at the pre-scanning stage and would tend to
> reserve it for the post scan stage.

That depends.  If you are getting high bit data out of the scanner, you are
fine doing it that way.  If you are getting 8 bit data out of the scanner,
then doing it pre-scan will do the tonal corrections to the high bit data in
the scanner BEFORE it converts it to 8 bit data...so you are better off
doing any major tonal correction pre-scan.  It also matters if it's B&W or
color.  8 bit color data, which is actually 24 bit color data BTW, can
tolerate "more" tonal correction than 8 bit B&W can.

As far as doing tonal corrections to high bit data, B&W or color, you are
fine doing it quite a few times, if not an infinite number of times,
depending on how drastic your corrections are.  The reason is you are only
going to be able to print a VERY small number of tones compared to the
number of tones in the file...so as long as you end up with a full histogram
of 8 bit data for B&W, and a mostly full histogram of 8 bit data for color,
you should be fine.

As far as doing tonal corrections to low bit data, you don't have much room
to do this with 8 bit B&W data, so I strongly suggest doing any B&W tonal
corrections to high bit data, or in the scanner driver which will always do
it to high bit data, even though it gives you 8 bit data out.

As far as doing tonal corrections to low bit color data, it's actually 24
bits of data, and it can withstand quite a bit of correction.

Regards,

Austin

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