[photo-3d] Subject: Re: Multiple flash...

[EDuw3D@xxxxxxx]

HOW TO CALCULATE GUIDE NUMBER FOR TWO FLASH UNITS

Guide Number (GN)for specific ASA is the distance ( feet or meters)   
multiplied by by the f stop.    What happens as you use two flashes  EXAMPLE: 
   IF you have a flash with a GN of 100  the f stop can be calculated by 
dividing the GN by the distance.    Let's say you are illuminating a object 
at 25 feet - the nearest f stop would be f 4  ---- ADD ANOTHER IDENTICAL 
FLASH  at the same location - and you OBVIOUSLY double the amount of light 
and you would close the lens opening down ONE f stop to F 5.6.      The guide 
number of the COMBINED  unit is then calculed by multiplying 5.6 times 25 
feet or  GN140.         To double the amont of light AGAIN,  you would need 
TWO MORE flash units  and the  F stop would be F 8 at ten feet with a GN of 
200.     

To  DOUBLE the guide number you quadruple the light out put of just the 
single flash.

The mathematical "fun" come when lighting a cave setting with flash bulbs at 
different distance from the camera AND  different  distances from the parts 
of the subject.       A  few years ago, I had a job shooting fisheye and 3D 
photos in  a cave system and many of the shots required 10 - 15  very large 
flash bulbs PER exposure.

Each set up required use of a distance range finder to help place the  
DIFFERENT GN bulbs and a lot of calcualtions to get the lighting as needed.   
Film may be cheap but large flash bulbs are expensive and the time to set 
them up is long.    GN works.     

EDDD

Ed Jameson
 3D Underwater Productions
 275 Randall Rd.
 P.O. Box 127
 Berlin, MA   01503-0127     USA
 Telephone  978-838-2683
 Email    EDuw3D@xxxxxxx
 

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