At 05:34 12/19/01, John Hudson wrote:
I was told today that any two different brands of studio strobes each
rated at 400 watts / second [or any other rating for that matter] might
not deliver the same intensity of light output. By way of analogy my
source said that in and of itself any particular strobe rating was like
saying that an automobile's engine had four, or five, or six cylinders
and so many valves but did not imply or specify the actual power output
other than suggest, for example, that a 800 w/s rating was likely to be
twice as intense as a 400 w/s rating.
Can any one out there explain just how you measure the intensity of
light output by reference to the watt per second rating and how this can
be converted into a guide number for any particular ISO film speed?
John Hudson
Sadly, there isn't any path from a Watt-second rating to a Guide Number.
I quoteth from AR-21, Kodak Master Photoguide, Electronic Flash Exposure
section:
"Watt-second ratings do not indicate light output."
The wheel in this section for finding aperture based on film speed and
subject distance uses both GN (guide number) and BCPS (beam candlepower
seconds) which are a direct measure of light output.
Watt-seconds and BCPS is analagous to engine displacement and horsepower
rating. An enormous engine displacement does not guarantee a high
horsepower rating. The Watt-second rating is only a very rough cut at how
much light the strobe can generate. The BCPS for two differnt models of
strobe having the same Watt-second ratings can easily be quite different;
enough to require different aperture settings!
In addition, there are too many other variables that affect actual light
output in the direction of the subject, such as type of bowl reflector on
the strobe (if any), and usage of soft boxes or umbrellas, etc. The GN (if
given for a strobe) is typically for ISO 100 film (in feet and/or in
meters) and must be converted to film speed being used.
Best method? Use a flash meter! There are some inexpensive ones
around. Once you have experience with various setups in a studio, you'll
know what aperture to use without the flash meter. A friend of mine has
power settings on his strobes marked, plus locations on his studio floor
marked for the light stands. He used a flash meter and shot some film to
calibrate and verify his setup(s).
-- John
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