the same guide number and an aperture of , the light source must be 5.5 meters or 18 feet from the subject. Alternatively, if one has an established flash-to-subject distance and wants to find the required f-number, one divides the guide number by the distance. Example: Guide number = 48 (m) and the distance is 6 meters; one needs and aperture of (GN 48 ÷ 6 m = ). Example for finding a distance Suppose a photographer wants to shoot with an aperture of and the guide number is 28 (m) / 92 (ft). The flash device must be 10 meters (33 feet) from the subject. In metric: GN 28 ÷ = 10 m In U.S. customary units: GN 92 ÷ = 33 ft Example for finding an aperture Suppose a photographer's flash-to-subject distance is 9.75 meters (32 feet) and the guide number is 39 (m) / 128 (ft). The aperture must be . In metric: GN 39 ÷ 9.75 m = In U.S. customary units: GN 128 ÷ 32 ft = Details The magnitude of guide numbers is a function of the following four variables: The total luminous energy (in lumen⋅seconds) emitted by the flash head (which is itself the product of the duration and the average luminous flux of a flash). See Glossary, below for illumination terminology. The solid angle subtended by the circular- or rectangular-profile beam as it leaves the flash head (the average of the beam's X- and Y-axis angles). The ISO sensitivity setting. Filters (either on the flash or on the camera lens). See Effect of filters, below. The above variables fall into two classes that influence the magnitude of guide numbers: Those that affect the non-distance-related intensity of a flash arriving at a scene (a property called illuminance, measured in lux) or its duration; namely its power setting, flash coverage angle, and color gels in front of the flash head. Those that affect the camera's non-aperture-related light sensitivity; namely lens filters and the ISO rating of the film/imaging sensor. Changing either the fstop or the flash-to-subject distance does not affect guide numbers because, by definition, choosing a different value for one factor is automatically accompanied by a reciprocal adjustment in the other. Most modern flash devices can operate at guide numbers less than their maximum ratings via either manual adjustment of their power settings in binned steps, by an in-flash automatic light sensing feature,