RE: Film scanner experiment & questions

["Hemingway, David J" <HEMINGD@xxxxxxxxxxxx>]

If you use the Raw 12 bit scan mode then he should get little or no
compression with the 4000.  From this it seems to indicate that you chose
the Color Negative profile and scanned at 12 bits.  This could clip a 3.0
density negative because a 3.0 density negative corresponds to about a
+1000000000 (you get the idea) stops overexposure.

David

-----Original Message-----
From: Gary L. Hunt [mailto:glh@datawav.net]
Sent: Friday, February 01, 2002 12:24 AM
To: scan@leben.com
Subject: Film scanner experiment & questions


Finally got around to trying to compare my Polaroid Sprintscan
4000 with my new Nikon SC4000.  I don't claim these results
mean a lot--if anything, I have more questions now than
before.  But I scanned a 21-step step wedge target (0.15 density
steps) negative in RGB mode on both scanners.  I varied the
exposure setting on the Polaroid and the analog gain on the
Nikon for 8-bit scans [to be sure I wasn't clipping the "highlights"],
and also scanned in hi-bit mode for both scanners.  I had the
vague idea this would at least let me use Photoshop to compare
the usable density range of the two scanners, and I guess it does,
but not so easily as one might think.

The target has a total density range of 3.0 (i.e. 0.05 to 3.05),
which is within the manufacturer's claims for "dynamic range"
for both scanners.  (Not that that means anything, as we have
been brought to know.)  And in fact neither scanner approaches
being able to distinguish the full range of the target even in high
bit mode.  The Nikon comes closer--by expanding the dense
regions with PS levels, I can separate the steps to about level
18 (19 in the green channel), although only by sampling them--the
histograms don't show distinct peaks beyond level 14 due to noise.
The 8-bit steps are detectable to maybe level 16 (17 in the green
channel), consistent with 8-bit resolution.  But even the 14-bit scan
only covers a density range of about 2.55-2.7.

The Polaroid numbers don't actually come out much different at
level 16/17 for 8-bit scans, but the 12-bit scan (where no scanning
adjustments are possible with Polacolor Insight) is extremely
compressed; nothing beyond level 14 or so (density 2.0) is detectable.
The change in step "density" is very pronounced in this mode, which
probably explains why my high-bit scans always looked so overly
dark.  In addition, the denser steps are very noisy--they look really
grainy.  (Actually they look this way for all the steps in this mode.)
Maybe my particular sample isn't working right, but it sure looks to
me like the 12-bit mode has less information than the 8-bit mode.

The Nikon scans are all much "smoother" (less grainy or noisy) looking.
(I used NikonScan 3.1 with all the processing options turned off.)  I also
made a scan with VueScan, which appears visually to have a very slightly
greater range in 8-bit mode.  This scan is smoother-looking yet, although
that could be due to some default setting in VueScan that I didn't notice.

Now for the questions:
(1) can anyone tell whether the density ranges I am getting with these 2
     scanners are in the ranges one might expect if everything was set
right?
     (Never mind the specs--I knew they were a pipe dream.)
(2) if not, is there something obvious I am missing in doing the scans?
(3) can someone suggest a reason why my 12-bit Polaroid scans are
     so compressed?
(4) any idea why the steps on the Polaroid scans look so much more grainy
     than the Nikon ones, even for relatively low density steps?

Gary Hunt <glh@datawav.net>

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