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)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-numberGuide numbers can be given in feet or meters, and are usually given for ISO 100 sensitivity." per
http://en.wikipedia.org/wiki/Guide_numberThe chart below is found here:
http://speedlights.net/speedlights-power-indexSpeedlights 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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