Guide number

The guide number for an electronic flash measures its wider area. ability to illuminate the subject to be photographed at a Guide numbers can be given in feet or metres, and are specific film or sensor sensitivity and .A usually given for ISO 100 sensitivity. The guide number higher guide number indicates a more powerful flash. The of 80 feet in the previous example corresponds to a guide guide number (or “GN”) can be used to compute flash number of approximately 24 metres. The calculations re- power, subject distance, or lens for cor- main the same: a target 6 metres (20 feet) away requires rect . an aperture of f/4 (24 = 6 × 4). Thus, when compar- For example, doubling the guide number means the flash ing flash units, make sure to compare the guide numbers, can illuminate an object at twice the distance, or for an measured in the same unit, for the same ISO rating and object at the same distance can be used at one quarter the the same “zoom” setting of the flash. sensitivity. Doubling the guide number requires a qua- drupling of the flash’s power, as the area to be illuminated increases as the square of the distance (cf. inverse-square 1.1 Distance law). The distance in the guide number calculation is the dis- Guide number is frequently abbreviated as “GN”.[1] tance from the flash to the subject. The position of the camera is not relevant. 1 Value The flash-to-subject distance is longer when bounced flash is used, and the illumination is less when a diffuser is attached to the flash unit. In the former case it is possible The guide number is the product of the maximum flash- to calculate the flash to subject distance that the light trav- to-subject distance and the f-number of the aperture that els along the bounce path instead, but this still does not will correctly expose film or a digital sensor with the spec- take into account the loss of light due to incomplete re- ified sensitivity. flection. Most photographers apply a rule of thumb such as opening up by one to two stops. However, nearly all GN = distance × f-number flashes available now have an automatic exposure capabil- ity that will automatically provide the correct illumination This simple relationship is well-defined because the regardless of diffuser or bounce use; the guide number is brightness of a flash falls off with the square of the dis- mostly useful for comparing the maximum light output tance, but the amount of light admitted through an aper- of several flash units. ture decreases with the square of the f-number. The guide number represents an exposure constant for a 1.2 Examples flash unit. For example, a guide number of 80 feet at ISO 100 means that a target 20 feet away will be correctly Typical built-in flash devices on may have a illuminated with an aperture of f/4 (80 = 20 × 4) using a guide numbers of 15 feet (4.6 m) or less, while high- sensitivity of ISO 100. For the same guide number and powered flashes can have guide numbers of over 250 feet an aperture of f/8, the light source should be 10 feet from (76 m).[3][4][5][6][7][8][9] the subject (80 = 10 × 8).[2] Guide numbers do not depend on the focal length of the lens: the distance a flash can illuminate does not depend 2 Applications on the angle of view of the lens. However, some flashes have the capability to “zoom with the camera” and con- centrate their light into a narrow beam for use with a tele- Guide numbers are especially useful in dealing with man- photo lens. Since the light from the flash is more concen- ual flash units when calculating the aperture for a particu- trated, this increases the guide number. Manufacturers lar exposure or the maximum flash range for a given aper- typically advertise the guide number for their flashes at ture. In practice, it’s normal to know the guide number the narrowest setting. For instance, the Olympus FL-50 and the distance and need the aperture, so the equation is has a guide number of 50m at ISO 100 when set to its nar- rearranged to give: rowest setting, but significantly less when illuminating a f-number = GN/distance

1 2 3 REFERENCES

Some flash units have a dial on the back (in fact, a simple form of slide rule) or a chart to help in calculating the desired settings. The aperture given by the guide number is only correct for certain locations, generally indoors where a reason- able amount of “stray” flash light will be reflected from surrounding walls onto the subject. The effective GN is slightly lower outdoors as any light not falling directly on the subject from the flash will be lost.

3 References

• http://strobecalc.eu5.org/

[1] “The Guide Number (GN) article at Photozone”

[2] Hobby, David (2007-12-21). “Guide Number: Your Free Meter”. Strobist.

[3] “Canon Speedlite 580EX”. GN of 190 feet (58m) at 105mm zoom, ISO 100.

[4] “Canon Speedlite 580EX II”. A GN of 58 (190ft) @ 105mm, ISO 100.

[5] “Metz mecablitz 76 MZ-5 digital”. A “potato-masher” style flash with a GN of 54 @ 50mm, or 76 @ 105mm zoom, ISO 100.

[6] “Nikon SB-400 Speedlight”. A Nikon flash for smaller cameras with a GN of 30m, “18mm zoom head position”, ISO 200.

[7] “Nikon SB-600 AF Speedlight”. A GN of 30/98 (ISO 100, m/ft), 42/138 (ISO 200, m/ft) @ 35mm.

[8] “Nikon SB-800 AF Speedlight”. A GN of 38m (125ft) @ 35mm, and 56m (184ft) @ 105mm, ISO 100.

[9] “Sunpak PF20XF”. A GN of 20, but no information about what angle of coverage that is measured at, ISO 100. 3

4 Text and image sources, contributors, and licenses

4.1 Text

• Guide number Source: http://en.wikipedia.org/wiki/Guide%20number?oldid=641200392 Contributors: Julesd, Pengo, Macrakis, Quadell, GreenReaper, Imroy, Mattdm, MisterSheik, Konetidy, RussBlau, RoySmith, Alai, LrdChaos, Melvin Pao, The wub, Srlef- fler, Caseman, YurikBot, Hede2000, Groogle, Gaius Cornelius, Rfugina, Stevage, Morio, Luigibob, CmdrObot, Thijs!bot, Oldman- biker, Bernopedia, JAnDbot, Tstrobaugh, Kauko56, Wolfram.Tungsten, Thegreenj, RenniePet, VolkovBot, SieBot, Addbot, Zorrobot, Fraulein451, 121D and ,درفش کاویانی ,AnomieBOT, ArthurBot, Ergodus, Itamarhason, AndyHe829, Phototutor, BG19bot, Reddogsix Anonymous: 27

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