Flash (photography)
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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 flash units as of 2009 are electronic, having evolved from single-use flash-bulbs 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 equipement, flashes may be large, standalone units, or studio strobes, powered by special battery packs or connected to 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.
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[edit] Types of flashes
[edit] Flash bulbs
The earliest flashes had 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. An innovation was coating flashbulbs with a blue plastic coating 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 it shattering during the flash. Later bulbs substituted zirconium for the magnesium, which produced a brighter flash and tended to blind people.
Flashbulbs took longer to reach full brightness and burned for longer than electronic flashes, and had slower shutter speeds (typically from 1/10 to 1/50 of a second) were used on cameras to ensure proper synchronization. A widely used flash bulbs 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.
[edit] Flashcubes, Magicubes and Flipflash
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, 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 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.
[edit] Modern flash technology
Today[update]'s flash units are often electronic flashtubes. 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 high-voltage flash units discharging 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, 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.
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.
[edit] Focal plane shutter synchronization
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 then 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 speeds 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 speeds above the shutter traverse time only part of the sensor is exposed. This limits the shutter speed to about 1/200s when using flash.
High-end flash units address this problem by offering a mode, typically called FP sync or HSS, 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.
In fact, 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.
[edit] Technique
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. [1]
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. [1]
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.
[edit] Drawbacks
- A typical problem with cameras using built-in flash units is the low intensity of the flash; the level of light produced will often not suffice for good pictures at distances of over 3 meters (10 ft) or so. Dark, murky pictures with excessive image noise or "grain" will result. In order to get good flash pictures with simple cameras, it is important not to exceed the recommended distance for flash pictures.
- The "Red-eye effect" is another problem. Since the retina of the human eye reflects red light straight back in the direction it came from, pictures taken from straight in front of a face often exhibit this effect. It can be somewhat reduced by using the "red eye reduction" found on many cameras (a pre-flash that makes the subject's irises contract). However, very good results can be obtained only with a flash unit that is separated from the camera, sufficiently far from the optical axis, or by using bounce flash, where the flash head is angled to bounce light off a wall, ceiling or reflector.
- Some cameras' flash exposure measuring logic fires a pre-flash very quickly before the real flash. In some camera/people combinations this will lead to shut eyes in every picture taken. The blink response time seems to be around 1/10 of a second. If the exposure flash is fired at approximately this interval after the TTL measuring flash, people will be squinting or have their eyes shut. One solution may be the FEL (flash exposure lock) offered on some more expensive cameras, which allows the photographer to fire the measuring flash at some earlier time, long (many seconds) before taking the real picture. Unfortunately many camera manufacturers do not make the TTL pre-flash interval configurable.
- Flash distracts people, limiting the number of pictures that can be taken without irritating them. Photographing with flash may not be permitted in some museums even after purchasing a permit for taking pictures.
[edit] See also
- BC Flash
- Guide number
- Flash synchronization
- Inverse-square law
- List of photographic equipment makers
- Ring flash
- Flash Comparison
- Flash-lamp
[edit] References
- ^ a b Langford, Michael (7th ed. 2000). Basic Photography. Focal Press/Butterworth Heinemann. pp. 117. ISBN 9780240515922. http://books.google.com/books?id=mj60n3uH4eMC&pg=PA198&dq=flash+lighting&lr=&client=safari#v=onepage&q=flash%20lighting&f=false.
[edit] External links
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