Re: [Leica] Extreme Low Light Color --> Mr Puts

["dwpost" <dwpost@email.msn.com>]

Andre-
I read with interest your post and would like to add what I know to the mix
and maybe help clear up the question.
As someone asked me once, after I had pontificated a bit, "Where did you get
your degree in psychology....?, I was forced to say, "Guilford College,
1972."
I was fascinated with perceptual psychology, being interested in
photography, and how the eye perceives color.
You seem to mistake the sensitivity of film with the sensitivity of film
when the mechanism, while depending on the photos hitting sensitive
surfaces, and making chemical changes, also has some distinct difference.
Our vision depends on light sensitive organs in the retina called rods, and
cones. One is used for low light vision, and are essentially monochromatic
in their response. The human eye therefore lacks color in very low light
situations- in extremely low light, we actually see in black and white. The
cones are sensitive to red, blue and green light, but require more energy to
activate, or simply, more light. The photosensitive chemical is a pigment
like material called rhodposin, or "Visual Purple" and the eye recycles this
chemical for the most part within the cells- you might think of it as sort
of a specialized neurotransmitter that once it does it job, and is
chemically altered, the eye tissue uptakes it and 'resets' its
photosensitivity.
Film, and you mentioned a negative film- a point that will become important
later- has usually three sensitive layers, and is some cases like the Fuji
Superia films, an additional layer. Theoretically, they should behave the
same at all light levels, but we all know about 'reciprocity failure'. Since
these several layers have different sensitivities, they behave in extremes
of light differently.
Therefore, in dim light, the rates of this reciprocity failure for the
different emulsions leads to what is known in photofinishing as 'color
failure'.
Your prints look greenish because of several interrelated factors- one, the
response of the film at low light with long exposures can cause the 'slopes'
of the individual layers to vary so much that one may predominate- result,
color shift!
The paper also has three emulsions with different response curves, and if
the exposure is too short- as in a very thin negative, or too long as in
printing a very dense negative, the 'slopes' or response curve may fall out
side the 'linear' portion of their response curve, and since all must be
balanced, you again get a color shift.
Photofinishers usually set up their equipment with standard test negatives
that represent a 1-2 stop underexposure, a normal, a 1-2 stop overexposure,
and if they are really dedicated, they set up using a super-overexposed
negative with about 5 stops overexposure. These setup programs mean that if
a certain film is printed, the channel is modified so that if the exposure
time is longer or shorter than normal, a filtration bias is added to correct
the balance.
Now, this works reasonably well, but when you factor in the paper response,
the film response, the sometime use of filters (Kodak's films often have
filter correction suggestions for exposures longer than 1/10 second....), as
well as the quality of the light under which the exposure was made, correct
color rendition is nearly impossible!
I would suggest that to maintain any semblance of quality and tonal
fidelity, that you use B&W film, and process it yourself. Most
photofinishing labs run the film through a processor that maintains a
constant development time, temperature and replenishment rate. Even if they
'push' the film, it is usually only about 2 stops at most.
If you use color, then you really ought to learn to do your C-41 film, learn
the limits of pushing it, and learn to print color where you can make the
usually huge color corrections necessary to get reasonable results, or the
exact tone you want.
All in all, I would say that B&W would be easier, and actually more like the
eye's low light, monochromatic vision. In any event, you will really have to
get to the stage where you do all your own processing since some really low
light stuff takes a bit of experimentation.
Good luck and hope you'll share your results with us.
Dan
- ----- Original Message -----
From: Andre Jean Quintal <megamax@abacom.com>
To: <leica-users@mejac.palo-alto.ca.us>
Sent: Saturday, December 18, 1999 8:22 AM
Subject: [Leica] Extreme Low Light Color --> Mr Puts

Clipped for brevity....