All about Flash
A flash is a device used in photography that produces an instantaneous flash of artifitial light (typically around 1/1000 to 1/200 of a second) at a color temperature of 5500 K aprox 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 synchronisation 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.
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 bulets tearing through objects like light bulbs or balloons
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. Many a times the guide number may actually be overrated !
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.
Technique
A flash is commonly used indoors as the main light source when there is not enough ambient light for a desired shutter speed. A fill flash or fill-in flash is a low powered flash added to ambient light to illuminate a subject close to the camera while using an exposure long enough to capture background detail. Another technique is to point a flash upwards onto a reflective surface, which may be a white ceiling or a flash umbrella, which reflects light onto the subject; this is called bounce flash. Bouncing creates a more natural light effect than direct flash without glare in the highlights and impenetrable shadows, 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 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.[citation needed]