filmscanners: Polaroid Optical Density

["Hemingway, David J" <HEMINGD@polaroid.com>]

I have done this before on this forum and hopefully you will find it use
full to understand how Polaroid determines optical density. I invite other
manufacturers to be as forthcoming!
The first thing to realize it the OD is NOT just a density reading, noise is
also factored.
Polaroid uses a custom designed target that contains a white patch and a
whole bunch at the higher density range. This target is expose in , I think,
Velvia which has the highest density of any E6 film available at the time
the test was designed. The actual density values as measured using a
densitometer or spectrophotometer. The target is then scanned and
statistical analysis is made of every pixel in each patch. As the density
gets higher the percentage of pixels that are noise increases. This is the
point where cheating can occur. Statistically how much noise you allow and
still accept the value as acceptable. You can literally take a single data
point and manipulate it to give substantially different OD specifications.
This is where multi-scanning comes in. I will now reveal one of my personal
gripes. Multi-scanning is generally perceived as "good" and a desired
feature. In my view it is there to compensate for poor design and/or
inferior components. Do I wish we had it, yes but only to satisfy the
"checkbox" buyer. Simply, multi-scanning attempts to identify the pixels
that change  from scan to scan, These pixels are most likely noise and when
statistically identified as such can be removed and replaced with values for
adjacent pixels. While multi-scanning can produce registration  problems,
more likely it will produce a slight softening of the image particularly at
higher DPI. Some time ago when Ed Hamrick was writing the driver for the
SS4000 he tried to implement multi-scanning and he said because of the low
noise level of the SS4000 multi-scanning produced little advantage.

In a practical sense let me relate my real word experiences with regards to
this subject. I work/attend trade show all over the world for Polaroid.
Invariably potential customers come up to me and ask if they can scan
"their" slide to look at the shadow detail. They then will try to do the
same at other manufacturers booths. In the three or more years I have been
doing this I have NEVER lost this type of shootout with a comparable
scanner.

To me the bottom line is :
1.      Do not make any decision based on manufacturers specifications
2.      If possible do the test yourself or rely on a good reliable
independent source.

Thanks for you time
David