I've found that showing this formula is easy for a beginner to grasp:
flash's guide number
Aperture- (F setting number) = ------------------------------
flash to subject distance
The guide number for a T20 flash when you're using 100 speed film is the
number "20", which refers to 20 meters. Divide the flash to subject distance
(this distance as expressed in meters) into the "20" (guide number).
So, for a subject 2 meters away, divide the "2" into the "20" and you get
"10". Set your lens to a aperture ("F" number) of F /11 (closest number to
"10")
If you'd rather use "feet" as a measurement unit, you must use your flash's
guide number in "feet" also. I think the T20's guide number in feet is 66. So
divide your roughly 6 feet distance into the guide number of 66, and you get
"11" for your answer. Set your lens to F/11
Always remember to set your flash to the film speed you're using. And you
must convert your flash's guide number when you change your film speed. ( The
guide number of 20 [in meters] is only used for 100 speed film).
George S.
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