P3D Guide number question

["Dr. George A. Themelis" <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

>I have a Vivitar Zoom Thyristor 2500.  Can anyone tell me the guide 
>number for this flash?  

It is very easy to tell the quide number of any flash if there is
any manual exposure information in the back.  For a given ASA value
muliply any distance with the suggested f-stop value.  For example,
if for ASA 100 the flash suggests that you use f8 at 7 feet then
the guide number is: 8 x 7 = 56 feet at ASA 100.  Look at the 
manual settings only (not automatic).  This product should be
(approximately) constant for any pair of distance & f-stop you select.

-- George Themelis


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