Guide Numbers

[Halthron]

In trying to figure out which strobe would best meet my needs, I've discovered guide numbers and wanted to make sure I understood their use properly.

GNs seem to be the best exposure given in appeture times distance at ISO 100. Rated in air, I assume that if underwater numbers are given that the water would be as clear as possible, like pool water.

Underwater numbers consistently seem to be about 1/2 those of air numbers. Is that a good assumption to make for those manuf. that don't give UW numbers?

So if I'm looking at a GN of 24, I can use 12 for the UW value. To determine how far it will illuminate, 12 (GN) divided by 2.8 (widest appeture) = 4.25 (distance) at ISO 100.

So if I'm willing to use ISO 400, I can take 4.25 and double it twice (two full stops) and get a distance of 17? Assuming gin-clear water, reducing for expected viz?

Assuming I understand the above correctly, what is the distance? Feet, meters or it depends on the manuf.?

How reliable are the GNs? For instance, looking at the guide numbers for the Ike DS-125 on the Ike website shows a GN of 32 while the info at DigitalDiver shows a GN of 20. Or is this one of those questions that shouldn't be asked?

 

[gert7to3]

Try this discussion link from a year ago:
http://www.scubaboard.com/showthread.php?t=94758&page=2

 

[bobf]

GNs seem to be the best exposure given in appeture times distance at ISO 100.

Agree. Guide Number equals aperture times distance....or....gn=a times d.......sensitivity default setting ISO 100, measured in either feet or meters depending upon strobe manufacturer's preference, strobe of course set to full dump. Here in the States, the default distance is 10 feet. Set up a target 10 feet from strobe to subject. Take a series of exposures. The aperture that yeilds the best exposure when strobe was set to full dump is then multiplied by 10 (distance) and the result of the multiplication is the published Guide Number. For instance if a strobe has a guide number rating of 110, then f/11 at 10 feet, ISO 100 gave the best results.

Underwater numbers consistently seem to be about 1/2 those of air numbers. Is that a good assumption to make for those manuf. that don't give UW numbers?

I'm not sure anything can be taken for granted. Lens design also plays an important role.

To determine how far it will illuminate, 12 (GN) divided by 2.8 (widest appeture) = 4.25 (distance) at ISO 100.

Maybe topside through air but not water. Even the most powerful strobes underwater lose their effectiveness beyond 6 feet due to the filtering characteristics water has on light. I assume your GN 12 and subsequent measurement of 4.25 would have been measured originally in meters. No commercially available strobe is capable of properly illuminating an underwater composition from a strobe to subject distance of 4.25 meters ( over 13 feet).

.......what is the distance? Feet, meters or it depends on the manuf.?

How reliable are the GNs? For instance, looking at the guide numbers for the Ike DS-125 on the Ike website shows a GN of 32 while the info at DigitalDiver shows a GN of 20. Or is this one of those questions that shouldn't be asked?

For uniformity, all Guide Number data found in the StrobeFinder is listed in meters, ISO 100. If you travel to any of the manufacturers' web sites, they will always include the ISO and measuring system used (feet or meters) if they supply the GN. Ikelite, manufactured in the USA, is one of the few that use feet. The StrobeFinder translated Ikelite's information into meters to conform with all other strobe manufacturers. Before publishing the StrobeFinder on the internet, Ike Brigham himself checked for accuracy the Ikelite data.

Along with Guide Number and Watt second ratings for strobes, also consider their angle of coverage. Some strobes emit relatively tight spots of light, which is quite acceptable for macro work. Those strobes may even have a relatively high guide number rating. But it would be very misleading to compare their GN rating to a strobe capable of projecting a much wider beam angle.

hth,
b