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An electronic flash has a maximum distance that it is capable of illuminating. There's also usually a close point where it will start to become too bright and overexpose the subject. The distance from the closest to the furthest point that it can illuminate is the flash range. The range can be increased by using a faster film or more sensitive CCD, and reduced by diffusing the light source using ND filters or material. The range quoted is usually based on using ISO 100 film or a CCD with equivalent sensitivity. APS cameras are, for some reason, quoted using ISO 200 film.
Many cameras now have a built-in flash that is used to take pictures inside when the light levels are low. Most cameras will have an Auto mode and there are usually several other modes including, Red eye reduction, Off, On, Slow sync.Most SLRs and some compacts have the option of attaching an external flash on an accessory shoe. Some have a sync socket so you can attach a studio flash off the camera for more controllable results. Buying advice: The flash range is often quoted and ones with more power will allow well-exposed pictures to be taken at greater distances. Having several modes increases the camera's versatility. If you would like to use studio flash you can fire heads remotely using your camera's built in flash and a slave unit, but watch out on digital cameras - the flash may not be synchronised correctly. It's better to use a camera with a sync socket. The most useful feature of an auto camera is Flash Off so you can avoid flash when you want to shoot in low light and Flash On to force the flash to fire to get good fill-in light in contrasty conditions.
Flash units often have a range of settings that controls the amount of flash emitted. With portable on-camera flash this is normally controlled automatically by either the camera or a sensor on the flashgun. Studio flash are often less sophisticated and have a slider or switch offering half or quarter power. Some of the more expensive models can be adjusted more accurately though a greater range of stops.