| Each film and flashbulb (or electronic flash unit) combination has a
Guide Number. This number will be found in the film instructions, flashbulb
package, or electronic flash instructions.
To find the proper lens opening, just divide the distance to the subject
into the guide number.
For example, a recommended guide number may be 80. If the subject is 10
feet away, just divide 80 by 10 to obtain the correct aperture setting (8
or f/8). If the guide number is 55 and the subject distance is 10
feet, then the correct lens opening would be 5.5 (use f/5.6).
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