Re: Only shades of gray...

From: Richard Knoppow <dickburk@ix.netcom.com>
Subject: Re: Only shades of gray...
Date: Tue, 08 Jun 2004 21:07:48 -0700

I wasn't going to get into this thread but since you are into details...

> Since the paper gradation is continuous the perceived
> separation of tones depends on the visual system and the
> things which affect it, like illumination level. There is
> really no such thing as a minimum difference in tone, at
> least in relation to anything practical in making good
> prints.

As you said, average density (or reflection/transmission luminance in
relation to incident luminance) of some area is practically speaking a
continuous quantity although individual constituents of the density is
discrete grains. This is a physical quantity, and should accompany
measurement error whenever experimental values are discussed. These
measurements are in units of candela per square meter or foot candles.

On the other hand, perceived quantity (brightness) is a psychophysical
quantity, and its accuracy is limited by the errors in our sensory
system, which is larger than errors due to decent physical measurements.

On brightness, various things are of importance, but in this
particular context, "difference limen" or "just noticeable difference"
are in question. These things are very well studied in vision
psychophysical community and published in their journals as well as
numerous textbooks. The JND depends on various factors including the
size of the targets, target separation, baseline luminance, duration
of the stimulus presentation, etc. The JND is measured as the minimum
difference in luminance that can be detected by visual system to a
certain probability criterion.

In average viewing conditions and typical density range of prints, the
print's range would translate to some 100 JND's or on that
order. Slides would have some 10x more. However, actual JND's vary at
dark end, midtone, and highlights and spacing would not be equal.

Analogously, sound intensity is a physical quantity, and loudness is
psychophysical quantity. Fletcher at Bell Labs in 1930's is also
analogous to Jones at Kodak Labs.

I don't know much about olfactory system. Together with gustatory
system, these chemosensory systems are much less understood
scientifically.

I don't think the work of Jones is considered obscure. There are
zillions of things that we should know better, and it's just one of
them :-)

Journal of the Franklin Institute is a fun reading, at least from
1930's and 1940's. I just learned that B-29 was basically a huge air
conditioner that happened to fly (well, not quite). That's the kind of
stuff that my dentist's waiting room should have. (Actually my dentist
has no waiting time because appointments are efficiently made online
or by email with no overlap, and he locks the door in other times.)

--
Ryuji Suzuki
"You have to realize that junk is not the problem in and of itself.
Junk is the symptom, not the problem."
(Bob Dylan 1971; source: No Direction Home by Robert Shelton)