Jadlupp@aol.com
Wed, 27 Oct 1999 12:55:59 -0400 (EDT)
In a message dated 10/26/99 5:02:18 PM Eastern Daylight Time,
jeffrey.d.mathias@worldnet.att.net writes:
<< The eye is
far more accurate in judging differences in density than a
densitometer. I recall an article from Scientific American (back in the
seventies) discussing the eye's capability to discriminate a
differential of three photons or some such small amount. >>
I think the reference is to the threshold of light against pure darkness, not
a variation between shades light or dark. I have an article in front of me
from a past issue of Shutterbug, written by Ctein (I don't know what issue, I
could only find a copy reduced down to 8 1/2 by 11 ) that discusses that the
eye does not see in continuous tones, only the brain integrates the
information and tells us we do. Test show that we can only see a two percent
difference in luminosity, or 0.006 desity units.
This means a young eye can see 650 steps from pure, blazing light to no light
at all. A trained artist's (or photographer's) eye may approach 900 steps.
The argument that the eye is sensitive enough for not needing a densitometer
for photo work is still correct (Ctein says if you can't see it, it doesn't
matter anyway), but not that the eye is necessarily more sensitive than a
densitometer.
I did find I could tell the difference on a print of .01 density where my
Stouffer 30 step tablet goes from 1 to 15 and 15 to 30. The two 15 steps were
different by .01, and the old Seagull Select FB paper would show a difference
in that range. I had the step tablet checked on three different
densitometers, and found they could each measure .01 difference. I could not
see it, because these steps on the tablet are 1/4 inch apart.
Ctein also goes on to talk about prints, where the maximum visual steps (for
the eye) is 250 when the print is viewed under 200 foot-candles of light, and
350 steps under 500 foot-candles. Room light is usually 20 to 40
foot-candles, where the darkest 25 percent of the image will lose 50 percent
of the steps. Track lighting, and sunlight, will restore the visual step.
Ctein gives two references, a 1980 Kodak book (out of print), and a 1975
article from the "Journal of Mathematical Biology."
Don
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