Flash (photography)

The high-speed wing action of a hummingbird hawk-moth is frozen by flash. The flash has given the foreground more illumination than the background. See Inverse-square law.

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 (a 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, called slaves.

Types of flash

Flash-lamp

Demonstration of a magnesium flash powder lamp from 1909

Studies of magnesium by Bunsen and Roscoe in 1859 showed that burning this metal produced a light with similar qualities to daylight. The potential application to photography inspired Edward Sonstadt to investigate methods of manufacturing magnesium so that it would burn reliably for this use. He applied for patents in 1862 and by 1864 had started the Manchester Magnesium Company with Edward Mellor. With the help of engineer William Mather, who was also a director of the company, they produced flat magnesium ribbon, which was said to burn more consistently and completely so giving better illumination than round wire. It also had the benefit of being a simpler and cheaper process than making round wire.[1] Mather was also credited with the invention of a holder for the ribbon, which formed a lamp to burn it in.[2] A variety of magnesium ribbon holders were produced by other manufacturers, such as the Pistol Flashmeter, which incorporated an inscribed ruler that allowed the photographer to use the correct length of ribbon for the exposure they needed. The packaging also implies that the magnesium ribbon was not necessarily broken off before being ignited.

Vintage AHA smokeless flash powder lamp kit, Germany

An alternative to ribbon was flash powder, a mixture of magnesium powder and potassium chlorate, introduced by its German inventors Adolf Miethe and Johannes Gaedicke in 1887. A measured amount was put into a pan or trough and ignited by hand, producing a brief brilliant flash of light, along with the smoke and noise that might be expected from such an explosive event. This could be a life-threatening activity, especially if the flash powder was damp.[3] An electrically triggered flash lamp was invented by Joshua Lionel Cowen in 1899. His patent describes a device for igniting photographers’ flash powder by using dry cell batteries to heat a wire fuse. Variations and alternatives were touted from time to time and a few found a measure of success, especially for amateur use. In 1905, one French photographer was using intense non-explosive flashes produced by a special mechanized carbon arc lamp to photograph subjects in his studio,[4] but more portable and less expensive devices prevailed. On through the 1920s, flash photography normally meant a professional photographer sprinkling powder into the trough of a T-shaped flash lamp, holding it aloft, then triggering a brief and (usually) harmless bit of pyrotechnics.

Flashbulbs

Ernst Leitz Wetzlar flash from 1950s
Flashbulbs have ranged in size from the diminutive AG-1 to the massive No. 75.
Kodak Brownie Hawkeye with "Kodalite Flasholder" and Sylvania P25 blue-dot daylight-type flashbulb
The AG-1 flashbulb, introduced in 1958, used wires protruding from its base as electrical contacts; this eliminated the need for a separate metal base.

The use of flash powder in an open lamp was replaced by flashbulbs; magnesium filaments were contained in bulbs filled with oxygen gas, and electrically ignited by a contact in the camera shutter.[5] Manufactured flashbulbs were first produced commercially in Germany in 1929.[6] 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 the coating of flashbulbs with a plastic film to maintain bulb integrity in the event of the glass shattering during the flash. A blue plastic film was introduced as an option to match the spectral quality of the flash to daylight-balanced colour film. Subsequently, the magnesium was replaced by zirconium, which produced a brighter flash.

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. Cameras with flash sync triggered the flashbulb a fraction of a second before opening the shutter, allowing faster shutter speeds. A flashbulb widely used during the 1960s was the Press 25, the (about 1 inch (25 mm) in diameter) flashbulb often used by newspapermen in period movies, usually attached to a press camera or a twin-lens reflex camera. Its peak light output was around a million lumens. Other flashbulbs in common use were the M-series, M-2, M-3 etc., which had a small ("miniature") metal bayonet base fused to the glass bulb. The largest flashbulb ever produced was the GE Mazda No. 75, initially developed for nighttime aerial photography during World War II.[7]

The all-glass PF1 bulb was introduced in 1954.[8] Eliminating both the metal base, and the multiple manufacturing steps needed to attach it to the glass bulb, cut the cost substantially compared to the larger M series bulbs. The design required a fibre ring around the base to hold the contact wires against the side of the glass base. An adapter was available allowing the bulb to fit into flash guns that accepted the bayonet capped bulbs. The PF1 (along with the M2) had a faster ignition time (less delay between shutter contact and peak output), so it could be used with X synch below 1/30 of a second—while most bulbs require a shutter speed of 1/15 on X synch to keep the shutter open long enough for the bulb to ignite and burn. A smaller version, the AG-1 was introduced in 1958 which did not require the fibre ring. Though it was smaller and had reduced light output, it was cheaper to manufacture and rapidly supplanted the PF1.

Flashcubes, Magicubes and Flipflash

Flashcube fitted to a Kodak Instamatic camera, showing both unused (left) and used (right) bulbs
Undersides of Flashcube (left) and Magicube (right) cartridges
"Flip flash" type cartridge

In the late 1960s Kodak improved their Instamatic camera line by replacing the individual flashbulb technology (used on early Instamatics) with the Flashcube. A flashcube was a single-use module with four flashbulbs mounted at 90° from the others in its own reflector. For use it was mounted on a swivel mechanism atop the camera that also provided an electrical connection to the shutter release and a battery inside the camera. After each exposure, the film advance mechanism also rotated the flashcube 90° to a fresh bulb. This arrangement allowed the user to take four images in rapid succession before inserting a new flashcube.

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 struck a primer tube at the base of the bulb, which contained a fulminate, which in turn ignited shredded zirconium foil in the flash. 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 look similar but are 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, the X-Cube.

Other common flashbulb-based devices were the Flashbar and Flipflash which provided about ten flashes from a single unit. The Flipflash name derived from the fact that once half the flashes had been used up, the unit was flipped over and re-inserted to use the remainder.

Electronic flash

Electronic flash was developed in the late 1950s while flashbulbs were still in general use. Early units were expensive and often large and heavy as the power unit was a separate unit to the flash gun head and was powered by a large lead-acid battery, and usually carried by the photographer by means of a shoulder strap. Towards the end of the 1960s, the size had dropped to the point where they were matching the size of conventional bulb flash guns (though the price, having dropped, was still an obstacle). The electronic flash system eventually superseded bulb guns as prices came down, rendering flashbulbs virtually obsolete.

A typical electronic flash unit has electronic circuitry to charge a high-capacity capacitor to several hundred volts. When the flash is triggered by the shutter's flash synchronization contact, the capacitor is discharged almost instantaneously through a flash tube, producing a flash of very brief duration almost instantaneously (i.e., the flash duration, often around 1/1000 of a second, is shorter than most practical shutter speeds, and full brightness is reached before the shutter has time to close appreciably). Synchronization of full flash brightness with maximum shutter opening was problematic with bulbs which took an appreciable time to ignite and reach full brightness; electronic flash does not have these difficulties.

Simple electronic flash units are often mounted on or near the camera; many inexpensive cameras have an electronic flash unit built in.

Two professional xenon tube flashes

Some lenses have built-in (ring-)flash lights for shadow free macro photography,[9] but there are also accessory ring flashes available.

In a photographic studio, more powerful and flexible studio flash systems are used. They usually contain a modeling light, an incandescent light bulb close to the flash tube; the continuous illumination of the modeling light lets the photographer visualize the effect of the flash. A system may comprise multiple synchronised flashes for multi-source lighting.

The strength of a flash device is often indicated in terms of a guide number designed to simplify exposure setting. The energy released by larger studio flash units, such as monolights, is indicated in watt-seconds.

The Nikon brand name for its electronic flash units, Speedlight, is frequently used as a generic term for electronic flash equipment.

High speed flash

An air-gap flash is a high-voltage device that discharges a flash of light with an exceptionally short duration, often much less than one microsecond. These are commonly used by scientists or engineers for examining extremely fast-moving objects or reactions, famous for producing images of bullets tearing through light bulbs and balloons (see Harold Eugene Edgerton). An example of a process by which to create a high speed flash is the exploding wire method.

A photo of a Smith & Wesson Model 686 firing, taken with a high speed air-gap flash. The photo was taken in a darkened room, with camera's shutter open and the flash was triggered by the sound of the shot using a microphone.

Multi-flash

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 Electric Research Laboratories (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.[10]

Flash intensity

Unlike flashbulbs, the intensity of an electronic flash can be adjusted on some units. To do this, smaller flash units typically vary the capacitor discharge time, whereas larger (e.g., higher power, studio) units typically vary the capacitor charge. Color temperature can change as a result of varying the capacitor charge, thus making color corrections necessary. Due to advances in semiconductor technology, some studio units can now control intensity by varying the discharge time and thereby provide consistent color temperature.[11]

Flash intensity is typically measured in stops or in fractions (1, 1/2, 1/4, 1/8 etc.). Some monolights display an "EV Number", so that a photographer can know the difference in brightness between different flash units with different watt-second ratings. EV10.0 is defined as 6400 watt-seconds, and EV9.0 is one stop lower, i.e. 3200 watt-seconds.[12]

Flash duration

Flash duration is commonly described by two numbers that are expressed in fractions of a second:

For example, a single flash event might have a t.5 value of 1/1200 and t.1 of 1/450. These values determine the ability of a flash to "freeze" moving subjects in applications such as sports photography.

In cases where intensity is controlled by capacitor discharge time, t.5 and t.1 decrease with decreasing intensity. Conversely, in cases where intensity is controlled by capacitor charge, t.5 and t.1 increase with decreasing intensity due to the non-linearity of the capacitor's discharge curve.

Flash LED used in phones

High-current flash LEDs are used as flash sources in camera phones, although they are not yet at the power levels to equal xenon flash devices (that are rarely used in phones) in still cameras. The major advantages of LEDs over xenon include low voltage operation, higher efficiency, and extreme miniaturization. The LED flash can also be used for illumination of video recordings or as an autofocus assist lamp in low-light conditions.

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 the second curtain follows it after a delay equal to the nominal shutter speed. A typical modern focal-plane shutter takes about 1/200 s 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 1/200 s 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 1/2000 s (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.

Technique

Image exposed without additional lighting (left) and with fill flash (right)
Lighting produced by direct flash (left) and bounced flash (right)

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.[13] Several techniques are used to soften light from the flash or provide other effects.

Drawbacks

The distance limitation as seen when taking picture of the wooden floor
Flash
The same picture taken with incandescent ambient light, using a longer exposure and a higher ISO speed setting. The distance is no longer restricted, but the colors are unnatural because of a lack of color temperature compensation, and the picture may suffer from more grain or noise.
No flash
Left: the distance limitation as seen when taking picture of the wooden floor. Right: the same picture taken with incandescent ambient light, using a longer exposure and a higher ISO speed setting. The distance is no longer restricted, but the colors are unnatural because of a lack of color temperature compensation, and the picture may suffer from more grain or noise.
Using a flash in a museum is mostly prohibited.

See also

References

  1. ↑ McNeil, Ian (2002). An Encyclopaedia of the History of Technology. Routledge. pp. 113–114. Retrieved 14 August 2014.
  2. ↑ Chapman, James Gardiner (1934). Manchester and Photography. Manchester: Palatine Press. pp. 17–18.
  3. ↑ Jayon, Bill. "Dangers in the Dark" (PDF). Archived from the original (PDF) on May 4, 2015. Retrieved 25 July 2014.
  4. ↑ "Taking instantaneous photographs by electric light". Popular Mechanics. 7 (2): 233. February 1905.
  5. ↑ Solbert, Oscar N.; Newhall, Beaumont; Card, James G., eds. (November 1953). "The First Flash Bulb" (PDF). Image, Journal of Photography of George Eastman House. Rochester, N.Y.: International Museum of Photography at George Eastman House Inc. 2 (6): 34. Retrieved 26 June 2014.
  6. ↑ Wightman, Dr. Eugene P. "Photoflash 62 Years Ago" (PDF). Image, Journal of Photography of George Eastman House. Rochester, N.Y.: International Museum of Photography at George Eastman House Inc. IV (7): 49–50. Retrieved 4 August 2014.
  7. ↑ Anderson, Christopher. "Photoflash bulbs". Darklight Imagery. Retrieved 23 October 2014. The largest flashbulb, the mammoth GE Mazda Type 75, was initially developed to be used as a source of light for night time aerial photography during world war II. The Mazda 75 measured over eight inches long and had a girth of 14 inches!
  8. ↑ http://www.flashbulbs.com/philips/ph-6.html
  9. ↑ For example, the Nikon Medical Nikkor Lens
  10. ↑ Nicholls, Kyle. "Non-photorealistic Camera". Photo.net. Retrieved 28 December 2011.
  11. ↑ "Studio Flash Explained: Flash Duration". Paul C. Buff, Inc. Retrieved 5 July 2013.
  12. ↑ "Einstein – User Manual/Operation Instructions" (PDF). Paul C. Buff, Inc. p. 13. Retrieved 5 July 2013.
  13. 1 2 Langford, Michael (2000). Basic Photography (7th ed.). Focal Press/Butterworth Heinemann. p. 117. ISBN 978-0-240-51592-2.
  14. ↑ "Stobe Tips". Addendum. June 12, 2010.
  15. ↑ George, Chris (2008). Mastering Digital Flash Photography: The Complete Reference Guide. Lark Books. pp. 102–. ISBN 9781600592096. Retrieved 14 January 2017.

Further reading

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