I have always used this formula for 35mm electronic flashes
GN/fstop x sqrt. (ASA/100) = distance
However, this formula does not take into consideration the fl lens being used... or the fl setting on the flash either. The formula assumes you enter the prope r GN for the fl lens you plan to shoot with (and also the equiv. fl setting on t he flash). Therfore when a Guide Number is quoted, it assumes a specific lens f l, some flash makers use 50mm and others like Cannon are more aggressive and use their high end zoom fl's, like 105mm. Some manuals produce tables showing the different GN's at each zoom level. However if I want to add a fresnel in front of the flash to decrease its angle of coverage to that equal to a 300mm lens, th en how do I figure out the new GN for that flash at 300mm vs. say 105mm? I coul d not find a relationship between GN and fl in the manuals... for example, at 35 mm fl the GN = 35, but at 105mm the GN = 54? I have never seen a formula take t he lens fl (or angle of coverage) into account? Can anyone introduce this varia ble into the formula?
I realize for LF I would simply convert the 35mm to LF equivalents ... but to k eep this question simple, lets just treat it as a 35mm issue. Any input would b e helpful.... Thank you in advance...
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