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Re: filmscanners: OT: Film grain



In a message dated 6/30/2001 11:15:54 PM Pacific Daylight Time,
kingphoto@mindspring.com writes:


C-41 film has so much latitude that manufactures can rate it one to
two stops faster than the optimal speed and get away with it.  


That's not exactly true.  Film speed is determined by methods established by
standards organizations; it's not up to the whim of an individual film
manufacturer.  For example, the International Organization for
Standardization (ISO) in conjunction with the American National Standards
Institute (ANSI) publishes standard ANSI/ISO 2240 for determining the speed
of transparency film.  There's a standard for determining C-41 film speed,
also, but I haven't read it.  But for transparency film, ANSI/ISO 2240 gives
the temperature and environmental conditions for film storage and processing,
states that it shall be exposed with a shutter speed between 5 seconds and
1/1000 second, and requires that amateur film be developed between 5 and 10
days after exposure while professional film is to be developed between 4
hours and 7 days after exposure (this is to mimic the conditions under which
such films are actually used).  Two densitometer readings are made, they are
combined in a mathematical formula to calculate the lux-seconds, and the
lux-seconds value is used in a lookup table to read the speed.  The table is
constructed in 1/3-stop speed increments.  Both arithmetic (ISO) and
logarithmic (DIN) speeds are determined by the table.  Everyone using
ANSI/ISO 2240 to determine film speed for a given film will get the same
answer, even a film maker testing a competitor's film.

That said, it is true that film manufacturers send representatives to the
various standards organizations, including ANSI, where the various standards
are set.  But so do camera manufacturers and even amateur photography
consumer groups (Photographic Society of America).  All of them together
reach a consensus as to the content of a standard.  In the case film speed,
they try to determine the optimum value and the standard is written
accordingly.  In the case of C-41 film, they've all evidently decided that
the optimum speed is the one that gives the highest speed while providing
adequate results.  You may disagree with their definition of optimum, but
that's how film speed gets determined.  Obviously, some films are a bit
quirky (Kodak's old VPS is a prime example, rated at ISO 160, shot at 125
with studio lighting, and shot at 80 with some outdoor lighting conditions)
and don't give the best results when shot at their ISO rating.  But others
(Kodak's current Portra films) are dead on at their rated ISO.  Sure, you can
over expose C-41 film a bit, even a Portra film, if you want to center it in
the middle of its latitude in order to compensate for improper metering
technique.  Overexposing a bit to center the film in its latitude so that you
could maximize your sloppiness in exposure control was usually of more value
to the amateur than the pro.  That might be different now that photo
manipulation software is so common since a pro might be able to shoot high
contrast scenes and then use something like Photoshop to control the contrast
in the final output.  Personally, I'd just as soon figure out how to control
the contrast in the scene beforehand so I get a proper negative/transparency
the first time, rather than have to rely on Photoshop to undo the damage done
by high contrast.  For one thing, I'd have a higher quality photo if I didn't
have to digitize it and I'd probably save a lot of time by controlling scene
contrast as opposed to doing damage control via digital manipulation.

In any event, that's how film speed is determined.  I hope you found it
enlightening.  At one time, I thought as you did, that the marketers at the
film companies were playing games with film speed.  When everyone started
shooting Velvia at 40 instead of its rated ISO 50, I did some research and
learned that there actually is a rigid standard for determining film speed.  
Of course, it doesn't work as well for some films, but it does keep
manufactures a bit more honest than if there were no standards at all.


 




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