Paul C. Buff, Inc. Technical Forum

Hi,

I'm just trying to know the exact power of the AB flashes with a guide number calculated in metres not in feet. Could you help me with that? It's the only way i could compare AB with other flashes.

Thank you so much

http://www.alienbees.com/specs.html Our specs for GN are listed here in feet. Multiply these by .3048 to get the GN in meters. (scroll down about 2/3 of the way).

However, be sure you are comparing apples to apples. You must compare the flashes using the same angle of reflector. Notice we have two GN's listed for each flash. Some manufactures will use a 50 degree reflector, some maybe 30 degrees. If you compare those to a flash units using the GN for an 80 degree reflector, you are putting that light at a disadvantage. You must also compare them at the same ISO. Some will use ISO 100 and some will use ISO 200. If you use a standard distance to calculate, then it is easy to calculate one stop difference (f/11 to f/8, for example) but if you use a standard aperture, then the distance would not be doubled, but multiplied by 1.414 (10 meters to 14.14 meters, for example).

The GN's are above, but be sure to have all variables the same, otherwise you will be giving yourself false information.

thanks for your help, this is a bit hard when you are a photographer without technical studies. it must be an standard that all the flashes use to talk about their light power!

i was comparing with a flash that have these info:

GN 90: 1m, 100 ISO, with reflector 48°

so, if i had understand it well, to compare apples with apples, an AB800 have a GN of 97 at 1m, iso 100 and a 50º reflector, isnt it?

i'm not sure because i cant understand the last part of your message "If you use a standard distance to calculate, then it is easy to calculate one stop difference (f/11 to f/8, for example) but if you use a standard aperture, then the distance would not be doubled, but multiplied by 1.414 (10 meters to 14.14 meters, for example)."

thank you so much

Your two comparisons are very close, about as close as you are going to get.

GN = aperture x distance. Since you are maintaining a constant distance (1m), then your comparisons are OK.

However, some people want to know how far back thier lights have to be to get a certain aperture (we will use f/8 for this example). If a GN is 80m @ ISO 100, then you can be 10 meters away and get f/8. If you double your sensitivity to ISO 200, you cannot double your distance to maintain f/8, you can only multiply it by 1.414. (so, 14.14m). You would have to go to ISO 400 to double the distance. For comparison of flash power, the important part is to make sure your ISO is constant. The rest is important for calculating exposure.

Are you comparing it w/ hotshoe type flash?
They "cheat" their guide number by quoting them at zoomed (concentrated beams) settings for their "max".
Now if you want powerful hotshoe flashes, look at the Metz potato mashers or the Agfatronic 643cs. They get real guide number over 100...the Agfa actually is 210 :-)

the 48 degree specification would specify zoom setting. While GN comparisons are not the absolute best way to compare, they are the most universally used.

i was comparing with an elinchrom bx500 ri with a 48º reflector