Guide number

What does the Guide Number mean that is associated with a particular strobe? ie, 12 or 20.

Assumption: camera set to ISO 100.

Divide the Guide Number by the distance to get f-stop.

Make sure the rating you use is an underwater Guide Number, sometimes it is rated as a GN in air that makes it look more brawny than it really is.

All the best, James

fdog always offers sound advise.

I would also add to make sure it is understood what measuring standard was used (feet or meters) in determining the published guide number. I wrote the following passage for the Strobe Finder in explaining Guide Numbers. Hope this helps.........

The guide number is one way of representing the power of the strobe, watt-seconds being another. The industry standard for determining a guide number top side (Air) is to place an object or target a distance of 10' from the strobe head. Next step is to capture the image of the object using an assortment of aperture openings while the strobe emits a full dump. The aperture which yields the best exposure is then multiplied by 10 (the distance from strobe to subject), resulting in the Guide Number. The ISO chosen is usually 100.

Although an underwater strobe is ineffective at influencing exposures underwater at a distance of ten feet, one could still determine an underwater guide number by following the same formula (GN=A*D). Place an object 3 feet from the strobe head. Set strobe for full dump and fire away, using various apertures. Once again, the aperture that yields the best exposure is 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 with the strobe set to full dump. IOW, the resulting image was neither overexposed or underexposed. 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 (ISO 100, feet).

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.

It's also important to point out that this is a only a guide...and bracketing is necessary as is any unique lighting you may want to try.

No reason to repeat what FDOG and IMSUSHI have already stated because they are right.