Guide number

When setting photoflash exposures, the guide number (GN) of photoflash devices (flashbulbs and electronic devices known as "studio strobes", "on-camera flashes", "electronic flashes", "flashes", "speedlights", and "speedlites") is a measure photographers can use to calculate either the required f-stop for any given flash-to-subject distance, or the required distance for any given f-stop. To solve for either of these two variables, one merely divides a device's guide number by the other. Though guide numbers are influenced by a variety of variables, their values are presented as the product of only two factors as follows: This simple inverse relationship holds true because the brightness of a flash declines with the square of the distance, but the amount of light admitted through an aperture decreases with the square of the f-number. Accordingly, as illustrated at right, a guide number can be factored to a just as readily as a . Guide numbers take into account the amount of luminous energy of the flash, the camera's ISO setting (film speed), flash coverage angle, and filters. Studio strobes in particular are often rated in watt⋅seconds, which is an absolute measure of illuminating power but is not particularly useful for calculating exposure settings.…

When setting photoflash exposures, the guide number (GN) of photoflash devices (flashbulbs and electronic devices known as "studio strobes", "on-camera flashes", "electronic flashes", "flashes", "speedlights", and "speedlites") is a measure photographers can use to calculate either the required f-stop for any given flash-to-subject distance, or the required distance for any given f-stop. To solve for either of these two variables, one merely divides a device's guide number by the other. Though guide numbers are influenced by a variety of variables, their values are presented as the product of only two factors as follows: This simple inverse relationship holds true because the brightness of a flash declines with the square of the distance, but the amount of light admitted through an aperture decreases with the square of the f-number. Accordingly, as illustrated at right, a guide number can be factored to a just as readily as a . Guide numbers take into account the amount of luminous energy of the flash, the camera's ISO setting (film speed), flash coverage angle, and filters. Studio strobes in particular are often rated in watt⋅seconds, which is an absolute measure of illuminating power but is not particularly useful for calculating exposure settings. All else being equal, a guide number that twice as great will permit subjects to be properly exposed from twice as far away or an f-number twice as great. The guide number system, which manufacturers adopted after consistent-performing mass-produced flashbulbs became available in the late 1930s, has become nearly superfluous due to the ubiquity of electronic photoflash devices featuring variable flash output and automatic exposure control, as well as digital cameras, which make it trivially easy, quick, and inexpensive to adjust exposures and try again. Still, guide numbers in combination with flash devices set to manual exposure mode remain valuable in a variety of circumstances, such as when unusual or exacting results are required and when shooting non-average scenery. Different models of flash devices available on the market have widely varying maximum-rated guide numbers. Since guide numbers are so familiar to photographers, they are near-universally used by manufacturers of on-camera flash devices to advertise their products' relative capability. However, such a practice demands industry-wide standardization of both the ISO setting and illumination angle underlying the ratings; this has only been partially realized. For the most part, manufacturers state guide numbers relative to a sensitivity of ISO 100. However, manufacturers sometimes rate guide numbers at ISO 200, which makes them 41% greater. The illumination angles underlying manufacturers' ratings vary greatly, which can make it particularly difficult to compare models. It is easy to use guide numbers to calculate either an aperture or a flash-to-subject distance. Suppose a photographer has a flash device with a guide number of , sets the camera's aperture to , and wants to know the required flash-to-subject distance; he merely divides the guide number by the f-number, 4. Thus, a subject away will be correctly illuminated at / 4 = . For the same guide number and an aperture of , the light source must be from the subject. Alternatively, if one has an established flash-to-subject distance and wants to find the required f-number, one divides the guide number by the distance. For example, with a guide number GN = at , one needs an aperture of : GN / = .

Example for finding a distance

Suppose a photographer wants to shoot with an aperture of and the guide number is . The flash device must be from the subject.

Example for finding an aperture

Suppose a photographer's flash-to-subject distance is and the guide number is . The aperture must be : GN / = . \text{Reduced power GN} = \text{Full power GN} \times \sqrt{ \text{Power ratio}}. For example, suppose a flash's guide number is . At power, the reduced-power GN is . \text{Filtered GN} = \text{GN} \times \sqrt{0.5^{n}} where n is the filter's rated loss in stops. The table to the right provides some common filter values. When a flash device is set to manual (M) or automatic (A) exposure mode and is not being controlled via the camera's through-the-lens metering, a convenient way to compensate for the effect of a lens-mounted filter is to set the ISO rating on a camera to a higher value than the flash device. For instance, if a polarizing filter attenuates by 1 stop and the flash device is set to ISO 100, then the camera can simply be set to ISO 200. The extra camera sensitivity compensates for the loss due to the filter. The formula governing this relationship is as follows: \text{Camera ISO} = \text{Flash ISO} \times 2^{n} where n is the camera filter's rated loss in stops. For example, suppose a filter attenuates by stops and the flash device is set to ISO 100. Then the filter factor , so the camera ISO should be set to times the flash ISO of 100, or about 252. The nearest standard camera setting is ISO 250. The GE stopped producing useful amounts of light roughly after current was applied. Thus, a camera with a fastest shutter speed of of a second (one that began exposures after a bulb was fired with M sync triggering), and which was set to of a second, would close its shutter after triggering a flashbulb (  +  = ) and would achieve the maximum rated guide number from the . So long as one used flashbulbs with leaf shutter-type cameras, faster exposures and larger apertures could be used to minimize motion blur or reduce depth of field at the expense of guide number. In the case of the GE Synchro-Press with M sync for instance, shutter speeds as long as of a second still diminished its guide number, though it still managed an impressive at a second exposure. This relationship between shutter speed and guide number was reflected in the guide number tables printed on flashbulb packaging after the industry-wide adoption of the guide number system, as exemplified by the below-left table for the . Cameras with focal-plane shutters even if they had PC connectors with X, F, M, or S-sync delays ("xenon sync" with zero delay and flashbulbs with peak delays of 5, 20, and ) could not be used at speeds that attenuated guide numbers with most types of flashbulbs because their light curves were characterized by rapid rise and fall rates; the second shutter curtain would begin wiping shut during a period of rapid change in scene illuminance, causing uneven exposure across the image area that varied in nature depending on exposure duration and the type of bulb. With the GE Synchro-Press for instance, a modern camera with a focal-plane shutter and X sync would require a shutter speed of of a second ( ) to obtain an even exposure across the entire image area—and a not-insignificant boost in the guide number by capturing all the luminous energy to the left of the peak. A notable exception to this limitation with focal-plane shutters was when using FP sync in combination with "flat peak" (FP) bulbs, which had rise times followed by broad, relatively level plateaus in their light output curves. The FP bulbs, like the GE , allowed extraordinary flexibility with shutter speeds, ranging from the slowest speeds on the dial to the fastest where only a narrow slit passed over the film, at the expense of guide number. Prior to GE's inverse of the squares innovation, photographers and publications—via tedious trial and error with different flashbulbs and reflectors—generated tables providing a large number of aperture-distance combinations. For instance, a 1940 edition (written too late to incorporate guide numbers) of the Complete Introduction to Photography by the Journal of the Photographic Society of America featured an exposure table for foil-filled flashbulbs, which is shown below. The parenthetical values in bold were not part of the original table; they show the equivalent guide number for each aperture-distance combination. The original table used the common nomenclature values for the apertures; the guide numbers shown here are based on the precise aperture values from the {\sqrt{2}}^{\ n} series. Note the scatter in the guide number values in each column; the data for the right-most flashbulb setup has over a three-quarter f-stop variation from high to low. The above table is for only one film speed. For end users, obtaining proper exposures with flashbulbs was an error-prone effort as they mentally interpolated between distances and f-stop combinations that weren't very accurate in the first place. Had the guide number system existed by this point, the above table would not have required the left-most column showing distances and would have required only one row (showing guide numbers) under each heading. By 1941, two years after GE introduced the guide number system, guide number ratings for products like the GE were being discussed in books like Flash in Modern Photography. By 1944, the 16th edition of Wall's Dictionary of Photography featured a guide number table. Perhaps so as to not intimidate readers, that table still showed numerous combinations of distances and apertures, but it also featured a new column showing the guide number that every cell in its row equalled. The guide number system underlying that table drove slightly finer increases, averaging a factor of \sqrt{2} each, from one distance to the next (6, 9, 12, 18, and 24 feet) so each step would be accompanied by definition by an increase in aperture of precisely one f-stop. Not surprisingly, the data scatter was as tight as mathematical rounding to the nearest foot permitted. By late 1949, authors catering to hobbyists were using guide numbers in articles in a routine fashion, as exemplified by the January 1950 issue of Popular Photography, as follows: Upon introducing the new inverse of the squares concept in 1939, General Electric initially referred to the new system as "Flash Numbers". Two years later, Flash in Modern Photography (1941) used the term "guide number" on page 47, on the very next page used the term "Flash Number" (title case), and later still used the term "flash number" (lowercase). Terminology was similarly mixed in the UK for years after the introduction of the guide number system; circa 1954, "Flash Factor", "Flash number" (and sometimes "Guide number") were in use.

Text derived from https://en.wikipedia.org/w/index.php?oldid=1329809850 under CC BY-SA 4.0; modifications have been made.