GN of studio strobe versus hotshoe strobe

I'm still confused about the GN of studio strobe and the GN of hotshoe flash. I

have an Alien Bee B800 and a Canon 580EX. For the B800, It's got a GN of 110-

160 when used with a 48" umbrella (http://alienbees.com/specs.html). While the

Canon 580EX has a GN of 190 when at 105mm. The Canon flash has a zoom head,

B800 doesn't. I know I'm not having the Canon flash bounce light off the same

umbrella to compare GN, but how do I compare the power of these 2 flashes. How

much more powerful is the B800? Thanks!

Use the B800 at full power aimed at a flash meter 10 ft away with the ISO on the meter set to 100. Take a reading. Do the same with the 580Ex and compare the readings.

 

If you don't have a flash meter then take a series of exposures with your camera at several f-stops using the same flash to subject and ISO as the test above and compare the exposed film taken with each flash.

The GN of the B800 is 320 with the 11" reflector on (direct, no umbrella). Probably a similar beam spread on the 580EX is the 50mm coverage--I believe the GN for the 580EX at that beam angle is something like 130 or so? Both the reflector and the zoom head are giving a 50 degree beam spread. Read the Alien Bee info just below the guide number ratings on the page you show. The standard 7" reflector has an 80 degree (wide angle) beam spread (equivalent on the 580EX to the 24-28mm angle), according to the info (because these fill an umbrella or softbox nicely).

 

However, you should follow Brooks' suggestion to form your own personal GN for the units, nevermind the beam angles, if you intend to only use the B800 with the standard reflector.

Alien Bees B800 is a 320 WS unit. Canon 580EX is 58 WS.

Hmm. My 580EX manual says that 58 is the Guide Number (in meters) but says nothing about W-s, that I have found.

Christopher Hartt >> That's not right. 580EX is not 58Ws. It's GN is 58m at 105mm zoom head.