What good is a GN in air if you are using the thing underwater?
Guide number ratings are also used to compare power ratings from one strobe to another. Since less variables exist in air than water, most all strobe manufacturers list an air GN for each strobe produced. Some strobe manufacturers though believe watt seconds are a more reliable measure in determining strobe power.
And if it is a GN in air, how do your convert it to underwater?
Wish it were that easy, Bob. But since testing procedures and lens designs vary by manufacturer, there is no one easy formula that exists to punch the numbers into.
That said,
If any of you would like to see a chart of guide number ratings (tested by the manufacturer) for most every underwater strobe available to the diving community, visit digitaldiver.net's Strobe Finder at:
http://www.digitaldiver.net/strobes.php
Once there, click on one of the quick query buttons, such as
optically triggered. Next you may also wish to read short explanations (click on the links!)I wrote along with Richard Perry and Ryan Canon on guide numbers and other strobe features such as energy ratings, color temps, beam angle coverage, preflash compatibility, etc. along with their relevance.
Meanwhile, it is not too difficult to determine an underwater guide number on your own for an underwater strobe. Simply place an object underwater 3 feet from the strobe head. Set strobe for full dump and fire away using various apertures. The aperture that yields the best exposure (not overexposed, not underexposed) is then multiplied by 3 (in this case, 3 was the Distance from strobe to subject).
Let's assume that an Aperture of f/8 produced the best quality exposure. Multiply three times eight and the resulting number could be therefore considered the underwater guide number.
Example:
GN=A*D
GN = f/8*3 = an underwater guide number of 24.
Of course the clarity of the water would play an important role in the equation. But we can assume that if a strobe manufacturer supplied an underwater guide number, the test was conducted in a filtered swimming pool or crystal clear Caribbean water.
In this age of digital cameras, histograms, and instant feedback, understanding the guide number formula may not be as relevant as it once was. We find ourselves today in an era when Jim Watt's mantra of Shoot, Review, Adjust, Shoot Again is the preferred method among many point and shoot enthusiasts. otoh, when things begin to go awry, or if you wish to maximize depth of field, or even manual controllers, a small amount of background knowledge of the guide number formula may be worth it's weight in pixels.
At least it was for Ansel Adams and crew as gert7to3 pointed out.......
hth,
b
