umber 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)http://www.scantips.com/lights/flashbasics1c.html
"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 http://en.wikipedia.org/wiki/Guide_number
The chart below is found here: http://speedlights.net/speedlights-power-index
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: http://speedlights.net
Speedlight power comparison
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