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Re: [OM] Colour Balance of Slide Film

Subject: Re: [OM] Colour Balance of Slide Film
From: Roger Wesson <roger@xxxxxxxxxxxxxxxxxxxxxxx>
Date: Tue, 14 Jan 2003 10:29:44 +0000

Very curious effect.  The pink corona certainly looks quite unnatural.

For the 1999 solar eclipse I was underneath thick cloud in Cornwall. For the 2001 eclipse conditions could not have been better in Western Zambia, and I took loads of photos... but my camera got stolen with the eclipse film still inside it. So, it's always nice to see proper pictures from these two eclipses!

Cheers,
Roger


As for the pink leader, I can tell you K200 definitely turns pink
when it saturates with bright light.  I found this out when
photographing the total solar eclipse in 1999.  The bright corona,
which is nearly pure white, had a pink ring around it at a certain
saturation point, but not at the highest level of overexposure.  Very
strange action.  I since switched to E200 for eclipses, which does
not do this at all.  I also use E200 when I want less contrast than
Kodachrome.  Under some lighting conditions (like bright sun and
shade in the same shot) I find Kodachrome's high contrast to be a
problem getting the right exposure.

For example of the pink corona with K200:
http://groups.msn.com/firstlightimaging/1999blackseasolareclipse.msnw?action=ShowPhoto&PhotoID=159

For example of a proper looking corona with E200:
http://groups.msn.com/firstlightimaging/2001africantotalsolareclipse.msnw?action=ShowPhoto&PhotoID=51

Cheers,
Don Shedrick

--- CyberSimian <CyberSimian2@xxxxxxxxxxxxxx> wrote:

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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