Re: Optical Density ?
Re: Optical Density ?
- Subject: Re: Optical Density ?
- From: bruce fraser <email@hidden>
- Date: Wed, 4 Sep 2002 15:42:44 -0700
At 6:22 PM -0400 9/4/02, Jim Rich wrote:
David,
<<Is there a different scale for scanners to that of traditional film
measurement ?>>
The short answer is no.
A few years ago when we were comparing scanners and their density ranges a
scanner vendor pointed out that the best way to measure the density range of
of their scanner was to use a special gray scale they had created.
Skeptically, I asked why and after some techno mubo jumbo (which was sheer
foolishness) they admitted to me that they did this to get a higher dmax
value for marketing the product. I think that scanner vendor has now gone
out of business.
I find that using either a carbon dye grayscale as well as an IT8 to measure
density and density range provides realistic density readings.
I'd just add that a dMax spec that doesn't include the RMS noise at
that dMax is fairly meaningless (which is basically every scanner
dMax spec I've ever seen).
Basically, you test dmax by scanning darker and darker samples until
you reach the point at which the scanner can no longer detect a
difference. But the point at which the signal becomes unusably noisy
usually occurs a good bit north of the no-detectable-difference
point. Some vendors are more conservative than others at estimating
the useful dMax, so one vendor's 3.8 may actually produce deeper
usable shadows than another's 4.0. YMMV...
Bruce
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