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In photography, a flash is a device that produces an instantaneous flash of light (typically around 1/1000 of a second) at a Color temperature of about 5500K 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

Electronic Flash

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 commercial 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. In professional studio photography, flashes often take the form of large, standalone units, or studio strobes, that are powered by special battery packs 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.

Flash bulbs

The earliest flashes consisted of a quantity of magnesium 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 plastic coating to improve spectral quality as well as providing protection from the rare occasion when a flashbulb would crack during a flash. One of the most widely used flash bulbs up through the 1960s was the number 25. This is the large (approximately 1 inch (25mm) in diameter) flash bulb often showed used by newspapermen in period movies, usually attached to a medium format bellows camera.

Modern flash technology

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.

Technique

A flash is commonly used indoors as the main light source, because there is not enough other light for a desired shutter speed. A fill flash is a low powered flash mixed with ambient light, and is often used to illuminate shadows on the side of a subject facing the camera. Another technique, bouncing a flash, involves pointing a flash upwards off of a surface, often a white celling, where it is reflected back onto the subject. Bouncing creates a more natural light effect and lessens shadows and glare but requires more flash power than a direct flash.

Part of the bounced light can be also aimed directly on the subject by "bounce cards" attached to the flash unit. That increases the efficiency of the flash and compensates shadows caused 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.**

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