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DPChallenge Forums >> Hardware and Software >> guide numbers for 580ex
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05/16/2008 06:40:38 AM · #1
the guide number given for the canon 580 EX11 is 58m at 100 iso does anyone know the guide numbers at other iso settings.
e.g 200, 400, 800, 1600
05/16/2008 10:42:33 AM · #2
Just calculate it. Its 58m at ISO 100, f/1.

So at ISO 100 its 58m / <f-stop number>

E.g. f/2 is 29m, f/4 is 14.5m, etc.

But at ISO 400, its 192m / <f-stop number>

So at f/2 its 96m, f/4 is 58m, etc.

If you want the general equation for the 580, its: distance = 58 meters(ISO used / 100) / (f-stop number)

For the 430ex, substitute 43meters for 58 meters.

BTW, the distance is from the flash, not the camera (which only matters when you use off-camera flash)

05/16/2008 10:58:45 AM · #3
thanks for that, but still a little confused, the method I have used for years ehen using flash photography is guide number divided by distance in feet to obtain my f stop.

I have the guide number for ISO100 as 58m = approx 190.

you mention guide number for ISO 400 as 192m where has this figure come from.
05/16/2008 11:17:35 AM · #4
Try this fun little calculator:
Lighting and DOF calculator
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