[photo-3d] Guide number calculations with multiple flash units

["Brown, Fritz - LABS" <brownf@xxxxxxxxxxxxxxxx>]

Ed Jameson recently posted how to calculate the guide number when using
multiple flashes.  His method,  however only works if you use multiple
flashes that have the same GN.  If you are like me and have a motley
collection of flashes, no two of which are alike,  you can still calculate
the effective guide number of a multiple strobe set up.  If all the flashes
are set up as a unit, grouped together and aiming at the same thing,  the
effective guide number is the square root of the sum of the squares of the
individual guide numbers.  To give an example assume that you are using two
flashes with guide numbers of 60 and 100.  The effective guide number is
calculated by sqrt((100)^2+(60)^2) = sqrt(10000+3600) = sqrt(13600) = 116.
This works for any number of flashes used as a unit,  just take the square
root of the sum of their squares.  Start altering the distance and incedent
angles of the flashes and your calculations start to get more complex and a
flash meter becomes invaluable.

-Fritz M. Brown

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