Guide number

[[File:Vivitar 285 guide numbers.jpg|thumb|right|300px|A scene will be properly illuminated for every combination of f-stop and distance shown on this flash device's exposure calculation dial, each of which equals the same guide number.

The guide number here (full power setting, ISO 100, and normal-angle coverage) is 37 for calculations made in meters (yellow arrow) and 120 for feet (orange). For instance, on the foot scale, × = 120, as do both × and × . In meters, × = 37 as do × and every combination between.]]

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.

Understanding guide numbers

Units of measure

thumb|right|302px|Guide numbers are a composite unit of measure comprising two factors: and . Guide numbers are expressed in units of 'f-number' times distance (i.e., meters or feet).

Throughout most of the world where the metric system (SI) is observed, guide numbers are expressed as a unitless numeric value like 34, even though they are technically a composite unit of measure that is a product of the unitless f-number and distance (in SI units of meters): . As such, guide numbers can be reduced either to distance in meters or to f-stops depending on how one uses the guide number in a calculation.

In US customary units, photographers typically measure distances in feet and require guide numbers scaled accordingly. To serve the US market, manufacturers of flash devices typically provide foot-based guide numbers and add nomenclature such as feet, ft, or the foot symbol to unambiguously denote that fact, e.g. Guide number: . Another common practice when flash devices are marketed in the US is to provide guide numbers which can be expressed in a variety of formats so distances and f-numbers may be calculated using either feet or meters, e.g. Guide number: / . Shutter speeds do not factor into guide number calculations with electronic flash and, for the most part, have no effect on exposures.

Guide numbers are not affected by scene reflectance. Guide numbers are a function of the illuminance and duration of a flash (a property called luminous exposure that have lux⋅seconds as their units of measure) at a scene as measured by an incident-light meter (pictured at right), not the amount leaving the scene.

The built-in reflected-light meters in cameras are not a definitive measure of exposure. For example, using a camera with a through-the-lens meter to photograph a park bench surrounded by sunlit snow underexposes the image, making the bench appear nearly black and the snow as dark as grass and foliage. This is because reflected-light meters are calibrated for an average scene reflectance of 18% and do not give good results for scenes with non-average reflectance. A gray card and light meter can be used for better calibration.

Guide number distances are always measured from the flash device to the subject; if the flash device is detached from the camera, the position of the camera is irrelevant. Furthermore, unless a flash device has an automatic zoom feature that follows the setting of a camera's zoom lens, guide numbers do not vary with the focal length of lenses.

When manufacturers of flash devices provide guide numbers ratings specified relative to ISO 200, this increases them by the square root of the difference, or a 41 percent increase relative to those given at ISO 100. With peak powers often between one and two million lumens, many young baby boomers chased after fairy-like retinal bleached spots (a symptom of flash blindness) for minutes after having their pictures taken at close distance with flashbulbs of the era.

If one wanted the benefit of all the light produced by a flashbulb (highest possible guide number), relatively long exposure times were required because most flashbulbs did not stop producing useful amounts of light until after electrical current was applied. The GE flashbulb for instance, was a Class M (medium peak) bulb, which were designed to produce peak luminous flux of after firing (see the graph at lower right). The was intended for leaf shutter-type cameras and M sync photoflash triggering, which gave M bulbs a head start by delaying the opening of the shutter so any given camera's fastest exposure time would be centered at the point (an delay for instance, for a definitional camera capable of second exposures, or ).

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 .

{| class="wikitable"

|+ Effect of shutter speed on guide number for the GE Synchro-Press #11 flashbulb (6- or 7-in polished relfector, M sync, ISO 100)

! scope="col" | Shutter speed (s)

! scope="col" | Guide number

! scope="row" | ≥

! scope="row" |

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.

History

thumb|right|240px|Before the introduction of flashbulbs, photographers used magnesium [[flash powder in a flash-lamp. The pneumatic shutter release cords of the era featured rubber bulbs the photographer squeezed to take a photograph.]]

General Electric introduced the guide number system in 1939 concurrently with the introduction of a compact, wire-filled flashbulb called the This compelling new way of easily and accurately calculating photoflash exposures was quickly adopted by manufacturers of a wide variety of photographic equipment, including flashbulbs, film, cameras, and flashguns.

The first flashbulb was introduced in 1925 and was filled with flash powder. In 1929, Sashalite Limited in London invented the "Sashalite" flashbulb, which was filled with a crumpled wad of aluminum foil so thin (about one-tenth the width of a human hair) it could not be picked up with fingers.

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 <math display="inline">{\sqrt{2^{\ n}</math> 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.

{| class="wikitable"

|+ 1940 exposure experiment with foil-filled flashbulbs

! scope="col" rowspan=2 | Distance (in feet)

! scope="col" colspan=2 | Small bulb in metal reflector

! scope="col" colspan=2 | Large bulb in metal reflector

! scope="col" colspan=2 | Small bulb in ordinary reflector

! scope="col" colspan=2 | Large bulb in ordinary reflector

! scope="col" | !! scope="col" |

! scope="row" |

| ||

! scope="row" |

| ||

! scope="row" |

| ||

! scope="row" |

| .3 ||

| ||

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 <math>\sqrt{2}</math> 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".

References

External links

D. 'n' A. Seaver: Conley cameras
Photographic Memorabilia: Flash Photography ~ History & ILFORD
Scantips.com: Understanding Flash Guide Numbers, plus guide number Calculator
Scantips.com: EV – Exposure Value (with EV table and EV calculator)
Sekonic.com: EV/Lux/FootCandle Conversion Chart

Understanding guide numbers

Units of measure

History

See also

References

Further reading

External links