Guide Number.....Eh?

[David Thompson]

Can someone explain to me what this means and why it should be so important to me when buying a flash??

I'm sorry if this is a repeat of an old post, if it is can someone kindly direct me to the old post??

Cheers :bang:

 

[arclight]

It's an indication of the power of the flash gun.

Double the guide number and the flash emits 4 times the light (However, Inverse Square Law applies therefore if you had a flash gun with twice the guide number of another then the one with the higher number would be able to illuminate a subject at twice the distance as that with the lower number).

 

[Steve Smith]

Guide number is usually for IS0 100. Divide guide number by distance (use same units for distance as guide number is given in) and this gives you the correct aperture.

e.g. Flash with guide number of 110' @ ISO 100 at a distance of 12' Aperture = 110/12 = 9.16 so half way between f8 and f11 would be about right.

If you don't want to bother with working all of this out then the guide number is just an indication of the relative power of flashes.



Steve.

 

[DiddyDave]

And in case it's not obvious still - the higher the number the more powerful the flash

In general - buy the best you can afford as if you're shooting well below full power you'll find it recycles pretty quick

Oh and this "Inverse Square Law applies" rarely applies in practise (unless perhaps you're outside at night photographing something flying above you

DD

 

[incapete]

At it's most basic it's just a scale to compare the flash power - a Jessops 360AFD has a guide number of 36, a Canon 540EZ has a guide number of 54 (can you see a pattern in their naming...) and will be 50% more powerful at full power.

A Canon 430EZ sits somewhere in the middle...

Also, even if you don't need the full power of your flash, the recycle time will be faster on a dropped down 540EZ than on a 430EZ at full power.


EDIT: beaten to it by Dave... must type faster!