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[OM] Colour Balance of Slide Film

Subject: [OM] Colour Balance of Slide Film
From: "CyberSimian" <CyberSimian2@xxxxxxxxxxxxxx>
Date: Thu, 9 Jan 2003 07:30:02 -0000
I was looking through some slides to select my "Old Age" TOPE entry, and I
was struck once again by something that I first noticed several years ago
(see below).  The slides were shot on Kodachrome 200 film:

o   I shoot Kodachrome because I want the archival permanence of Kodachrome
(immunity to fading over decades).

o   I shoot 200ASA speed because 64ASA is too slow to handhold when I want
reasonable depth of field and am using a polariser.

What I noticed was the colour balance of the film leader.  That is the bit
of film that is completely fogged when loading the film, and so should be
completely transparent (this is slide film!).  Placing the leader on a white
sheet of paper shows that the leader has a slight magenta cast.  Numerous
films have shown this over the years (all Kodachrome), so I don't think that
this is due to improper storage or airport X-ray machines.  Some thoughts:

(1)  The entire film might have a slight magenta cast.  However, the human
eye/brain is notorious for correcting for colour casts and making things
look "normal", so a non-expert might think that the slides are OK (they look
OK to me), whilst an expert might notice the slight colour cast.  If most
people don't notice the colour cast, Kodak might have decided that this was
an acceptable compromise to make when designing the film.  If the entire
film DOES have a colour cast by design, it implies that one could use a
colour-correcting filter permanently on the lens to obtain slides with
neutral colour balance.

(2)  Most reasonably-knowledgeable photographers are familiar with the
concept of reciprocity failure.  Colour films suffer from differential
reciprocity failure, that is, each colour layer in the emulsion fails
reciprocally at a different rate.  The result is that exposures that are
very long (seconds) or very short (1/10,000 second) acquire colour casts.
One could regard the film leader as a very-long exposure, although since the
film is completely exposed there might be other effects coming into play.
However, it may be that completely exposed film also acquires a colour cast.
If the colour cast affects only long exposures or gross over exposure,
correctly exposed frames might have neutral colour balance, and hence NOT
need a colour-correcting filter.  If this is true, the colour balance of the
film leader cannot be used to deduce anything about the colour balance of
correctly exposed frames.

So, which do you think is the correct explanation -- (1) or (2) or something
else?

-- from Cy in the UK


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