Digital BW, The Print

Kevin,
 
my comments follow:
 
> I have trouble visually setting the WP and BP accurately on a raw scan
because it looks so compressed. 
 
I rely on numbers, i.e. open Photoshop levels and press AUTO. You can
configure this tool not to clip anything, so that only values with no data
are excluded. This way you're sure to keep all information from the scan,
and are free, later, if you feel like, to clip at either end of the
histogram.
 
> By setting the clip points to 0 vuescan sets the darkest part of the
negative in preview frame as the white point and the clearest part of the
negative in the preview frame as the BP.
 
I work with the raw scan, upon which most Vuescan settings, WP and BP
included, have no effect.
 
> I wonder if  the exposure actually increased by or if it is a software
compensation.
 
Considering that changing exposure affects the raw (unprocessed) scan, I'd
say it's a hardware thing.
 
> Do you know if the Epson 3200 can change the intensity of the light source
or the duration of the time the CCD is capturing info? From memory the RGB
exposure is a multiplier not like the analog gain in some of the Nikon
scanners that actually change the exposure time.
 
I expect all scanners to be able to change either of these parameters. My
gut feeling is that what changes is time rather than intensity, and I'm
pretty sure this is what Vuescan's RGB exposure does. It shouldn't be hard
to verify, though: just see how long it takes to scan the same image with
different values. If it's just a multiplier (I assume you mean a software
transformation of data from the raw range [x, y] to [n*x, n*y]) it should
take about the same time.
 
> I can't figure out why changing the actual exposure would do anything
other than move the range of values left or right in the histogram.
 
I think it works the same in a wet darkroom: take a correct exposure and a
very short exposure (say 1/4 of the previous) of the same piece of film, and
measure the difference in density between the lightest and darkest part of
each resulting print. I excpect this delta to be greater in the correct
print.
 
Alessandro

-----Original Message-----
From: Kevin Gulstene [mailto:kevin@...]
Sent: Tuesday, May 27, 2003 17:33
To: DigitalBlackandWhiteThePrint@yahoogroups.com
Subject: Re: [Digital BW] Re: Scanning


Allesandro-

Interesting.


> That's what I used to think and do, too, but very recently I changed my
> mind.
> These days I'm scanning Portra400BW 6x7 with an Epson3200 using 
> Vuescan, and
> my very simple workflow was to get a raw scan, extract the green 
> channel and
> Photoshop away, setting BP and WP etc.
> I was a bit concerned, though, on the huge amount of stretching I had 
> to do
> setting BP/WP in order to bring the usually *very* flat scan to cover 
> the
> whole 0-255 range, but given that I was starting from the raw scan,

The raw data is all crammed up against one end of the histogram.  
Somewhere along the workflow is needs to be 'stretched' by setting the 
white and black points.  You can do it in photoshop or the scanner 
software can do it.

I have trouble visually setting the WP and BP accurately on a raw scan 
because it looks so compressed.  I use vuescan to do that initially for 
me.  By setting the clip points to 0 vuescan sets the darkest part of 
the negative in preview frame as the white point and the clearest part 
of the negative in the preview frame as the BP.

> I
> thought I had no control over it. I was wrong: I found that the 
> automatic
> exposure set by Vuescan is, at least with this film-scanner 
> combination,
> usually way too short: talking Vuescan numbers, the automatic setting 
> is
> just below 4, the optimal setting ranges from 6 to 11 (you can 
> verify/change
> this number by checking the Lock exposure checkbox at the bottom of the
> Device tab: the value is set after the preview).
> I used to think that increasing exposure would only shift the histogram
> curve from the light to the dark end, but it's not true. Assuming that 
> the
> response is linear (I'll stand corrected on this: Austin, your 
> knowledged
> opinion is?), if exposure x gives a 20-100 range, exposure 2x gives 
> 40-200,
> thus resulting in a less flat scan. The result is a smoother image 
> (nothing
> outstanding, but noticeable), especially after steep curves and levels 
> (e.g.
> to bring dull skies to life)

I wonder if  the exposure actually increased by or if it is a software 
compensation.  Do you know if the Epson 3200 can change the intensity 
of the light source or the duration of the time the CCD is capturing 
info?  From memory the RGB exposure is a multiplier not like the analog 
gain in some of the Nikon scanners that actually change the exposure 
time.

I can't figure out why changing the actual exposure would do anything 
other than move the range of values left or right in the histogram.  If 
the RGB exposure in vuescan is just a multiplier then what you describe 
makes sense.

> Bottom line, my workflow now includes a little extra step at the 
> begininng,
> which is one or more 100dpi scan, checking the resulting histogram in
> Vuescan, with a locked exposure, until I get a reasonable range.


--
Kevin Gulstene
http://www.dockwalker.com <http://www.dockwalker.com> 



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