A flash is a device used in photography that produces an instantaneous flash of artificial light (typically around 1/1000 to 1/200 of a second) at a color temperature of about 5500 K to help illuminate a scene. While flashes can be used for a variety of reasons (e.g., capturing quickly moving objects, creating a different temperature light than the ambient light) they are mostly used to illuminate scenes that do not have enough available light to adequately expose the photograph. The term flash can either refer to the flash of light itself, or as a colloquialism for the electronic flash unit which discharges the flash of light. The vast majority of flash units today are electronic, having evolved from single-use flash-bulbs and flammable powders.
In lower-end consumer photography, flash units are commonly built directly into the camera, while higher-end cameras allow separate flash units to be mounted via a standardized accessory mount bracket often called a "hot shoe". In professional studio photography, flashes often take the form of large, standalone units, or studio strobes, that are powered by either special battery packs or connected directly to the mains and synchronized with the camera from either a flash synchronization cable, radio transmitter, or are light-triggered, meaning that only one flash unit needs to be synchronized with the camera, which in turn triggers the other units.
The earliest flashes consisted of a quantity of magnesium flash powder that was ignited by hand. Later, magnesium filaments were contained in flash bulbs, and electrically ignited by a contact in the camera shutter; such a bulb could only be used once, and was too hot to handle immediately after use, but the confinement of what would otherwise have amounted to a small explosion was an important advance. A later innovation was coating flashbulbs with a blue plastic coating to match the spectral quality to daylight balanced colour film as well as providing protection from the rare occasion when a flashbulb would crack during a flash. Later bulbs substituted zirconium for the magnesium which produced a brighter flash.
Flashbulbs took longer to achieve full brightness and burned for a longer duration than electronic flashes, and slower shutter speeds (typically from 1/10 to 1/50 of a second) were used on cameras to ensure proper synchronization. One of the most widely used flash bulbs up through the 1960s was the number 25. This is the large (approximately 1 inch (25 mm) in diameter) flash bulb often shown used by newspapermen in period movies, usually attached to a press camera or a twin-lens reflex camera.
In the late 1960s, Kodak improved their Instamatic camera line by replacing the individual flashbulb technology (used on early Instamatics) with the Flashcube. Flashcubes consisted of four electrically fired flashbulbs with an integral reflector in a cube-shaped arrangement that allowed taking four images in a row. The flashcube automatically rotated 90 degrees to a fresh bulb upon advancing the film to the next exposure.
The later Magicube (or X-Cube) retained the four-bulb format, and was superficially similar to the original Flashcube. However, in the Magicube each bulb was set off by a plastic pin in the cube mount that released a cocked spring wire within the cube. This wire, in turn, struck a primer tube, at the base of the bulb, which contained a fulminating material. The fulminate ignited shredded zirconium foil in the flash and, thus, a battery was not required. Magicubes could also be set off by inserting a thin object, such as a key or paper clip, into one of the slots in the bottom of the cube.
Flashcubes and Magicubes are superficially similar but not interchangeable. Cameras requiring flashcubes have a round socket and a round hole for the flashcube's pin, while those requiring Magicubes have a round shape with protruding studs and a square socket hole for the Magicube's square pin. The Magicube socket can also be seen as an X, which accounts for its alternate name, X-Cube.
Another common flashbulb-based device was the Flipflash which included ten or so bulbs in a single unit. The name derived from the fact that once half the flashes had been used up, the unit had to be flipped and re-inserted to use the remainder.
Today's flash units are often electronic xenon flash lamps. An electronic flash contains a tube filled with xenon gas, where electricity of high voltage is discharged to generate an electrical arc that emits a short flash of light. (A typical duration of the light impulse is 1/1000 second.) As of 2003, the majority of cameras targeted for consumer use have an electronic flash unit built in.
Another type of flash unit are microflashes, which are special, high-voltage flash units designed to discharge a flash of light with an exceptionally quick, sub-microsecond duration. These are commonly used by scientists or engineers for examining extremely fast moving objects or reactions, famous for producing images of bullets tearing through objects like lightbulbs or balloons (see Harold Eugene Edgerton).
Studio flashes usually contain a modeling light, which is an incandescent light bulb placed close to the flash tube. The continuous illumination of a modeling light helps in visualizing the effect of the flash.
The strength of a flash device is often indicated in terms of a guide number, despite the fact that the published guide numbers of different units can not necessarily be directly compared.
Although they are not yet at the power levels to replace Xenon flash devices in still cameras, LEDs (specifically, high current flash LEDs) have recently been used as flash sources in camera phones. LEDs are expected to approach the power levels of Xenon in the near future and may replace built-in Xenon flashes in still cameras. The major advantages of LEDs over Xenon include low voltage operation, higher efficiency and extreme miniaturization.
Part of the bounced light can be also aimed directly on the subject by "bounce cards" attached to the flash unit which increase the efficiency of the flash and illuminate shadows cast by light coming from the ceiling. It's also possible to use one's own palm for that purpose, resulting in warmer tones on the picture, as well as eliminating the need to carry additional accessories.
Some camera manufacturers may be considering the inclusion of a built-in bounce flash within the body of a camera with automated features to assist the user in obtaining a bounced light effect without spending time to set up and direct external flash devices.
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