A flash is a device used in photography producing a flash of artificial light (typically 1/1000 to 1/200 of a second) at a color temperature of about 5500 K to help illuminate a scene. A major purpose of a flash is to illuminate a dark scene. Other uses are capturing quickly moving objects or changing the quality of light. Flash refers either to the flash of light itself or to the electronic flash unit discharging the light. Most current flash units are electronic, having evolved from single-use flashbulbs and flammable powders. Modern cameras often activate flash units automatically.
Flash units are commonly built directly into a camera. Some cameras allow separate flash units to be mounted via a standardized "accessory mount" bracket (hot shoe). In professional studio equipment, flashes may be large, standalone units, or studio strobes, powered by special battery packs or connected to mains power. They are either synchronized with the camera using a flash synchronization cable or radio signal, or are light-triggered, meaning that only one flash unit needs to be synchronized with the camera, and in turn triggers the other units.
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The earliest flashes had of a quantity of flash powder consisting of a mechanical mixture of magnesium powder and potassium chlorate that was ignited by hand. Later, the electric flash-lamp used an electrical circuit to trigger a fuse to ignite explosive powder (e.g., magnesium) The flash-lamp was invented by Joshua Cohen (a.k.a. Joshua Lionel Cowen of the Lionel toy train fame) in 1899. The patent describes a device for igniting photographers’ flash powder by using dry cell batteries to heat a wire fuse.
The flash-lamp was replaced by flash bulbs; magnesium filaments were contained in bulbs filled with oxygen gas, 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. An innovation was the coating of flashbulbs with a blue plastic to match the spectral quality to daylight-balanced colour film and to make it look more moderate, as well as providing shielding for the bulb in the unlikely event of its shattering during the flash. Later bulbs substituted zirconium for the magnesium that had produced a brighter flash but tended to temporarily blind people.
Flashbulbs took longer to reach full brightness and burned for longer than electronic flashes. Slower shutter speeds (typically from 1/10 to 1/50 of a second) were used on cameras to ensure proper synchronization. Some shutters triggered the flashbulb a fraction of a second prior to opening, permitting use of faster shutter speeds. A flashbulb widely used through the 1960s was the number 25. This is the large (approximately 1 inch (25 mm) in diameter) flashbulb 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. A mechanism in the camera automatically rotated the flashcube 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, the Magicube did not require electrical power- 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 fulminate. The fulminate ignited shredded zirconium foil in the flash and, thus, a battery was not required. Magicubes could also be fired 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[update]'s flash units often incorporate electronic flashtubes, and are sometimes called speedlights or (erroneously) strobes. 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 is the microflash, a high-voltage device that discharges 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 light bulbs or balloons (see Harold Eugene Edgerton).
Studio flashes usually contain a modeling light, an incandescent light bulb 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. The strength of larger studio flash units, such as monolights are indicated in watt-seconds.
A camera that implements multiple flashes can be used to find depth edges or create stylized images. Such a camera has been developed by researchers at the Mitsubishi Electronics Research Laboratory (MERL). Successive flashing of strategically-placed flash mechanisms results in shadows along the depths of the scene. This information can be manipulated to suppress or enhance details or capture the intricate geometric features of a scene (even those hidden from the eye), to create a non-photorealistic image form. Such images could be useful in technical or medical imaging.[1]
Unlike flashbulbs, the intensity of an electronic flash can be adjusted on some units. Small flash units typically vary the length of time of the discharge, larger (studio) units typically charge the capacitor less. The disadvantage of charging the capacitor less is that the color temperature may change, necessitating corrections. With advances in semiconductor technology, there are some monolight (studio) units which can vary the time of the discharge.[2]
The flash duration is typically described with two numbers: t.5 is the length of time for which the flash impulse is above 0.5 (50%) of the peak intensity, while t.1 is the length of time for which the impulse is above 0.1 (10%) of the peak[3] (t.3 of course, would be above 30%). For instance, t.5 can be 1/1200 sec whereas t.1 can be 1/450 sec for the same flash at the same intensity. For a small flash controlling intensity by time, the t.5 and t.1 numbers decrease as the intensity decreases. On flash units controlling intensity by capacitor charge, the t.5 and t.1 numbers increase as the intensity decreases (i.e. takes longer for the capacitor to discharge to that point). These times become important if a person wants to freeze action with the flash (as in sports).
Flash intensity is typically measured in stops or in fractions (1, 1/2, 1/4, 1/8 etc.). Some monolights display an "EU Number", so that a photographer can know the difference in brightness between different flash units with different watt-second ratings. EU10.0 is defined as 6400 Watt-seconds, and EU9.0 is one stop lower, i.e. 3200 watt-seconds.[4]
Although they are not yet at the power levels to replace xenon flash devices in still cameras, high-current flash LEDs have recently been used as flash sources in camera phones. The major advantages of LEDs over xenon include low voltage operation, higher efficiency, and extreme miniaturization.
Electronic flash units have compatibility issues with focal-plane shutters. Focal-plane shutters expose using two curtains that cross the sensor. The first one opens and the second curtain follows it after a delay equal to the nominal shutter speed. A typical modern focal-plane shutter takes about 1/200s to cross the sensor, so at exposure times shorter than this only part of the sensor is uncovered at any one time. Electronic flash can have durations as short as 50 µs, so at such short exposure times only part of the sensor is exposed. This limits the shutter speed to about 1/200s when using flash. In the past, slow-burning single-use flash bulbs allowed the use of focal-plane shutters at maximum speed because they produced continuous light for the time taken for the exposing slit to cross the film gate. If these are found they cannot be used on modern cameras because the bulb must be fired *before* the first shutter curtain begins to move (M-sync); the X-sync used for electronic flash normally fires only when the first shutter curtain reaches the end of its travel.
High-end flash units address this problem by offering a mode, typically called FP sync or HSS (High Speed Sync), which fires the flash tube multiple times during the time the slit traverses the sensor. Such units require communication with the camera and are thus dedicated to a particular camera make. The multiple flashes result in a significant decrease in guide number, since each is only a part of the total flash power, but it's all that illuminates any particular part of the sensor. In general, if s is the shutter speed, and t is the shutter traverse time, the guide number reduces by √s / t. For example, if the guide number is 100, and the shutter traverse time is 5 ms (a shutter speed of 1/200s), and the shutter speed is set to 1/2000s (0.5 ms), the guide number reduces by a factor of √0.5 / 5, or about 3.16, so the resultant guide number at this speed would be about 32.
Current (2010) flash units frequently have much lower guide numbers in HSS mode than in normal modes, even at speeds below the shutter traverse time. For example, the Mecablitz 58 AF-1 digital flash unit has a guide number of 58 in normal operation, but only 20 in HSS mode, even at low speeds.
As well as dedicated studio use, flash may be used as the main light source where ambient light is inadequate, or as a supplementary source in more complex lighting situations. Basic flash lighting produces a hard, frontal light unless modified in some way.[5] Umbrellas and softboxes are commonly used for this purpose (even with small hand-held flash units).
Fill flash or "fill-in flash" describes flash used to supplement ambient light in order to illuminate a subject close to the camera that would otherwise be in shade relative to the rest of the scene. The flash unit is set to expose the subject correctly at a given aperture, while shutter speed is calculated to correctly expose for the background or ambient light at that aperture setting.
Bounce flash is a related technique in which flash is directed onto a reflective surface, for example a white ceiling or a flash umbrella, which then reflects light onto the subject. It can be used as fill-flash or, if used indoors, as ambient lighting for the whole scene. Bouncing creates softer, less artificial-looking illumination than direct flash, often reducing overall contrast and expanding shadow and highlight detail, and typically requires more flash power than direct lighting.[5]
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.
Also, slave flash units exist that are set up away from the subject and camera, that are triggered by the light from the master flash. This slave flash provides fill or bounce light. Many small flashes and studio monolights have optical slaves built in. Wireless radio transmitters, such as PocketWizards are also popular for remote synchronization since the receiver unit can be around a corner, or well over 100 meters away (which would be far too difficult to trigger using an optical sync).
Another method that can be used is strobe. Some high end units can be set to flash a specified number of times at a specified frequency. This allows action to be frozen multiple times in a single exposure.[6]
Colored gels can also be used to change the color of the flash. Correction gels are commonly used, so that the light of the flash would be the same as the tungsten lights (using a CTO gel) or the fluorescent lights.