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[filmscanners] HP SC-7400



RE: HP ScanJet-7400

Gentlemen..
I am new to this interesting group .. and have a question about HP's
ScanJet-7400, a 2400 ppi scanner.  I am currently using an HP SJ-6300
(1200 ppi).

Has anybody in the 'filmscanner' community evaluated the SJ-7400? Any
Q60 scans?  My interest in scanning includes 35mm, 120 and 220 film and
slides -- in addition to document and picture scans (TIFF and JPEG
formats mostly), mostly for business use.

Also, is there a document that explains in practical terms the "Dynamic
Range" parameter that is so frequently cited in discussion for film
scanning ?  How does one compute (or correlate) "D" rating to grayscale
histogram data as used in film scanners?  Is a 4.8D rating like "200%
clean"?

I've read some discussion of this in the last e-mail distributions.  I
do know a little something about "dynamic range" as we use it -- since
my work touches on 'machine vision' applications of imaging.  The
quality of the CCD chip is probably the major component of dynamic range
(in addition to the supporting electronics) -- the better the CCD (and
the more expensive) -- the more uniform the expected response of the
individual pixels in the array; moreover, the greater the potential
grayscale depth 8bit, 10bit, 12bit , etc. for 256, 1024, 4096, etc.
shades, respectively-- the better the scanned image quality.  With a
high quality CCD and support electronics these values can be approached
-- except at the low values (deep shadows) where the pixels don't
respond in a linear manner and spurious signal (black noise) results in
the loss of varying degrees of detail.  Similarly there may be a
non-linear response in the high-end.  Another issue may be in color
where the R, G and B response levels may diverge (e.g. composite "gray"
has a color cast), and don't track linearly (remember that the R, G and
B channels are "gray scale" that are viewed through respective color
filters).

I don't know if this adds to clarity or confusion, but this is the way
we tend to look at these issues in machine vision -- with the luxury of
working mostly with structured light for illumination, instrument
quality CCD cameras for image capture, and "midrange" grayscales for
image analysis via highly specialized software. And if I can find a way
to write a histogram file in TIFF, I'll share it for looking at the
crosssection of the grayscale on the bottom of the Q60 for some of the
scanner slides from your site.

Sincerely,

Frank K-F

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