Guide Number(s) and 580ex2

Ok sorry if seems like a dumb question, but i was taught when i was young that there isn't a dumb question .

Anyways, I know the max GN for the 580ex2 is 58 at 105zoom.
I read that it is also 28 at 28 zoom.

How do i figure out what the GN is at different zooms?
Is there a formula for it?

Thanks in advance.

Don't know about a formula but here's a link to the info.

http://photonotes.org …580exii&flash2=​*&flash3=*

Thank you so much.

The guide numbers can also be found in the flash owner's manual.

GN is typically measured with distance (so you sometimes have it in metres or feet). Basically when you "zoom" out with the flash, the light will not be as concentrated on one spot as it is on the 105mm setting, hence the light will travel much further in comparison to your light being spread out for a 28mm setting.

Most specifications are something like 58m @ 105mm (ISO 100). This means that you can get a correct exposure if your subject is 58 metres away from your camera with (ideally) no ambient light (ok maybe it's not that realistic if you can focus so far away in a dark room, but these are just measurements here), provided that you are on a 105mm setting on your flash and also that you are using ISO 100.
Why you don't see this in practice is that the measurement is based on an F-stop of 1. So if you were using an f-stop of 4, and you are flashing directly, then place yourself no more than 14.5m away. Also, the GN will increase linearly as you increase your ISO.

Hope this helps

ST87 wrote in post #6245688
GN is typically measured with distance (so you sometimes have it in metres or feet). Basically when you "zoom" out with the flash, the light will not be as concentrated on one spot as it is on the 105mm setting, hence the light will travel much further in comparison to your light being spread out for a 28mm setting.

Most specifications are something like 58m @ 105mm (ISO 100). This means that you can get a correct exposure if your subject is 58 metres away from your camera with (ideally) no ambient light (ok maybe it's not that realistic if you can focus so far away in a dark room, but these are just measurements here), provided that you are on a 105mm setting on your flash and also that you are using ISO 100.
Why you don't see this in practice is that the measurement is based on an F-stop of 1. So if you were using an f-stop of 4, and you are flashing directly, then place yourself no more than 14.5m away. Also, the GN will increase linearly as you increase your ISO.

Hope this helps

Well i already knew all that ... but thanks anyway.

PacAce wrote in post #6245531
The guide numbers can also be found in the flash owner's manual.

Yah, i just don't have access to my manual right now.
I forgot about checking photonotes.
oh well thanks again.