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Tips and Tricks:
Tip 4: Guide Numbers (continued)
Note: The concept of guide
numbers is valid for both land and underwater photography, and is
only used when shooting in manual mode. In other words,
the strobe is set to full power, the shutter speed is fixed, and
only the aperture is varied.
Guide numbers are a way of
rating the strength of a strobe's output.
Essentially it is a ratio between aperture and subject distance,
using ISO as a reference point. If this seems incomprehensible,
let me give you an example...
Suppose your strobe has a guide
number of 32 at ISO 100. That's the important
part: Guide number 32 with ISO 100. If you want to illuminate a
subject 4 feet away, set the aperture on your lens to 8. Why?
Because 32 divided by 4 equals 8. If the division doesn't work out,
choose the closest value. If the subject is 3 feet away set the
aperture to f11 (32 divided by 3 is 10.6)
I'm not suggesting anything as
difficult as mentally calculating fractional division underwater.
Consider the previous example. If the subject is 3 feet away, and
the strobe's guide number is 32, you know that dividing the two
will give you 10 and some fraction. F11 is the closest number to
10. That's all there is to it.
In an age of digital
sophistication I'm often asked why
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