Re: filmscanners: Polaroid Optical Density

["Maris V. Lidaka, Sr." <mlidaka@ameritech.net>]

Well put, David.  Thank you.

Maris

----- Original Message -----
From: "Hemingway, David J" <HEMINGD@POLAROID.COM>
To: <filmscanners@halftone.co.uk>
Sent: Sunday, August 26, 2001 1:32 PM
Subject: filmscanners: Polaroid Optical Density


| 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
|