filling in shadows outdoors, powerful flash devices (those with inherently greater guide numbers when compared at the same ISO sensitivity and coverage angle) can be useful because they permit photographers to increase the maximum flash-to-subject distance, such as when taking group photos. Clearly, more power helps because the sun is such a bright, unadjustable light source against which a flash device must compete. However, a contributing factor is that many modern cameras with focal-plane shutters can sync with flash devices at speeds no faster than of a second; such a relatively long exposure requires particularly small apertures and/or low ISO sensitivities, both of which constrain how far a camera and its on-camera flash can be from the subject. The tables below show the distances at which deep shadows will be filled with one fstop less than the sunlit portions of the scene, which is a common fill level. The exposures given here assume average frontlit subjects under bright or hazy sun with distinct shadows for photos taken between 2 hours after sunrise and 2 hours before sunset. As can be seen by referring to the above tables, if a photographer has a camera with an X-sync speed of 1/125 th of a second, is shopping for a flash device, and desires the capability to fill shadows from up to 2.5 meters (8 feet) away, a flash device with a guide number of at least will be required. Note that increasing the ISO sensitivity of a digital camera (or choosing a faster film speed) will not increase the distances given in these tables because the extent to which ISO sensitivity affects the exposure in the fill-flash areas of an image will equally affect the sunlit ones. For any given shutter speed, a flash device can only fill shadows to 50% the contribution from the sun out to a certain distance; at a camera's Xsync shutter speed, no change in ISO sensitivity—and accompanying fstop setting—can influence this proportional relationship. History General Electric introduced the guide number system in 1939 concurrently with the introduction of a compact, wire-filled flashbulb called the This compelling new way of easily and accurately calculating photoflash exposures was quickly adopted by manufacturers of a wide variety of photographic equipment, including flashbulbs, film, cameras, and flashguns. The first flashbulb was introduced in 1925 and was filled with flash powder. In 1929, Sashalite Limited in London invented the "Sashalite" flashbulb,