"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?"
This is the result of the way pro strobes are sold. They are generally sold
without reflector, unless a packaged special. Therefore, a guide number
would be only good for bare bulb. Most pros use a variety of light
modifiers, generally leaning toward soft boxes. The variety of uses means
that a flash meter is essential. Watt-seconds, by the way, indicate the
amount of energy sent to the flash tube, as I understand it. Let's not even
go into aging flash tubes.
I've noticed that a lot of portrait photographers use some sort of GN
system, although it is usually a system they have made up specifically for
their lights and studio, involving marks on the floor. But then, most
protrait photographers don't change lenses a lot.
I've never seen a commercial shooter use anything but a flash meter, and
lots of polaroid, a kind of belt and suspenders deal.
Bill Pearce
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