For those who don't already know or could not discover the guide # for this flash either in the instruction manual, which one has to download as a PDF, or easily on Google, the "40" is SF-40 indicates the guide # in Meters. So the guide # in feet is appx. 131 and I assume that might be for the 50 mm lens setting/coverage or maybe even for a 35mm lens, covered with the 24 mm setting on the flash. The formula would be 39.37 inches (a meter) divided by 12 inches (a foot) times 40 = guide # 131 in feet.
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