This simple calculator works out the required ƒ number setting for a camera lens from two known criteria. In the first form field box enter the given Guide Number of your flash gun at ISO 100. In the second form field box enter the distance the subject is away from the flash gun in Meters. Click on the calculate button and the results should appear below!

NOTE: Due to the nature of the way the formulas work the numbers generated could result in a decimal figure, for example an answer of 11.3. In this case the nearest available ƒ number on the camera lens would be the one to use and would probably be ƒ11.

Also, data must be entered in to each box even if it is a "0" (zero), else the "NaN" (Not a Number) error condition displays!

Still working in Feet and Inches with your distance to subject measurements? Not to worry use this simple converter to get the equivalent in meters for entry in the aperture calculator above!

NOTE: Data must be entered in to each box even if it is a "0" (zero), else the "NaN" (Not a Number) error condition displays!

You may have noticed that the Aperture Calculator requires the Guide Number to be entered as given at ISO 100 this is the usual standard ISO for stated flashgun Guide Numbers. However, if you find your guide number to be given at a different ISO rating you can use this calculator to find out the new Guide Number value at ISO 100.

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