When setting photoflash exposures, the guide number (GN) of photoflash devices (flashbulbs and electronic devices known as "studio strobes", "on-camera flashes", "electronic flashes", "flashes", and "speedlights") is a measure photographers can use to calculate either the required fstop for any given flash-to-subject distance, or the required distance for any given fstop. To solve for either of these two variables, one merely divides a device's guide number by the other. Though guide numbers are influenced by a variety of variables, their values are presented as the product of only two factors as follows: Guide number = f-number × distance This simple inverse relationship holds true because the brightness of a flash declines with the square of the distance, but the amount of light admitted through an aperture decreases with the square of the f-number. Accordingly, as illustrated at right, a guide number can be factored to a small fnumber times a long distance just as readily as a large fnumber times a short distance. Guide numbers are not an absolute measure of flash power (a property called luminous energy) because they are influenced by other variables, notably the camera's ISO setting (film speed) and flash coverage angle. For any given ISO setting and coverage angle however, an inherently more powerful flash device featuring a guide number that is, for instance, twice as great will permit subjects to be properly exposed from twice as far away for any given fstop, or conversely, will permit scenes at a given distance to be properly exposed at an fnumber twice as great. The guide number system, which manufacturers adopted after consistent-performing mass-produced flashbulbs became available in the late 1930s, has become nearly superfluous due to the ubiquity of electronic photoflash devices featuring variable flash output and automatic exposure control, as well as digital cameras, which make it trivially easy, quick, and inexpensive to adjust exposures and try again. Still, guide numbers in combination with flash devices set to manual exposure mode remain valuable in a variety of circumstances, such as when unusual or exacting results are required and when shooting non-average scenery. Different models of flash devices available on the market have widely varying maximum-rated guide numbers. Since guide numbers are so familiar to photographers, they are near-universally used by manufacturers of on-camera flash devices to advertise their products' relative capability. However, such a practice demands industry-wide standardization of both the ISO setting and illumination