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FAQ: How do Popular Speedlights Compare for GN?
Jan 22, 2013 13:51:17   #
Nikonian72 Loc: Chico CA
The Guide Number of a speedlight determines how powerful, or better yet, how far a speedlight will illuminate with enough power to make a proper exposure.

Correct GN is calculated with an international formula, in either feet or meters, and often both.
For variable angle speedlights, a 35-mm flash angle is used to calculate true GN.

Understanding Flash Guide Numbers (Including GN Calculator)

"The Guide Number is the product of the maximum flash-to-subject distance and the f-number of the aperture that will correctly expose film or a digital sensor with the specified sensitivity. GN = distance × f-number
Guide numbers can be given in feet or meters, and are usually given for ISO 100 sensitivity." per

The chart below is found here:
Speedlights NOT on the chart:
The popular Yongnuo YN-560 III has a mfr's listed GN of 39 at 35mm & ISO 100. Independent testing is closer to GN 34.
The newer Yongnuo YN-568EX is GN rated the same as a YN-565, which is GN 36.
The Canon 550EX and the Yongnuo 600 EX-RT have mfr's listed GN of 36 at 35mm & ISO 100. Independent testing measured GN at 29.

More detail about individual speedlights can be found here:

Speedlight power comparison
Speedlight power comparison...

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