How do you compare guide numbers?

Hi,

I am trying to compare the power of a few different flashes I have. From what I understand, higher Guide Number = More Powerful.

Here is where I am confused, do you need to look at the Guide Number @ the same distance (mm) for each flash for them to be comparable, or do you just compare max GNs across the board?

Here are the numbers I am trying to compare:
-Canon 430EX, GN of 141ft @ ISO 100, 105mm
-Promaster FTD 7000M, GN of 138ft @ ISO 100, 105mm
-Minolta Maxxum 4000 AF, GN of 148ft @ ISO 100, 70mm

So can I compare these GNs directly, or do I need to look at the GNs of the first two flashes @ 70mm to have them be comparable?

Thanks.

Two guide numbers can represent the relative power of two flash units if, and only if:
1) They use the same distance unit (meters vs. feet)
2) They use the same ISO (ISO 100 is most common)
3) They represent the same coverage angle (generally expressed as focal length on a 35mm camera).

Flash units with zooming heads have a different GN for each zoom setting.
To compare the power of your Minolta flash, lookup the GN for the 430EX and Promaster at 70mm. For the 430EX it's 37 meters (122 ft).

But the thing is... if you're using direct flash with a lens longer than 70mm, the 430EX and the Promaster can zoom more and give you more range. If you're bouncing or diffusing the light then that extra zoom won't help.

So it all depends on how you're using it.

Curtis N wrote in post #8299808
Two guide numbers can represent the relative power of two flash units if, and only if:
1) They use the same distance unit (meters vs. feet)
2) They use the same ISO (ISO 100 is most common)
3) They represent the same coverage angle (generally expressed as focal length on a 35mm camera).

Flash units with zooming heads have a different GN for each zoom setting.
To compare the power of your Minolta flash, lookup the GN for the 430EX and Promaster at 70mm. For the 430EX it's 37 meters (122 ft).

But the thing is... if you're using direct flash with a lens longer than 70mm, the 430EX and the Promaster can zoom more and give you more range. If you're bouncing or diffusing the light then that extra zoom won't help.

So it all depends on how you're using it.

Thanks Curtis, that was helpful. I'm pretty happy the Minolta is so powerful, because I got it for free, haha. The part about the extra zoom makes sense too, thanks for that tidbit.

Comparing the information in the OP, the Minolta with GN148 even at a relatively wide 70mm is the most powerful of the three!

Wilt wrote in post #8314879
Comparing the information in the OP, the Minolta with GN148 even at a relatively wide 70mm is the most powerful of the three!

I know how awesome is that! And I got it for free because I found it with some of my parents' old camera gear.

jcw122 wrote in post #8315555
I know how awesome is that! And I got it for free because I found it with some of my parents' old camera gear.

I presume you understand the limitations of trying to use it with your Canon body. Auto flash non-ETTL, or Manual flash. But take care about using the flash in your hotshoe, since the contacts are likely not all compatible!

Wilt wrote in post #8317557
I presume you understand the limitations of trying to use it with your Canon body. Auto flash non-ETTL, or Manual flash. But take care about using the flash in your hotshoe, since the contacts are likely not all compatible!

I do understand the limitations. I intend to actually use it in an off-camera setup triggered by Cactus V4s. I am also aware of hotshoe voltages, I haven't looked up the Minolta yet but I know the Promaster is safe. Nevertheless, I don't plan on using them on the hotshoe. Thanks anyway, always appreciated!