Guide number

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, f/4 × 30 ft = 120, as do both f/8 × 15 ft and f/16 × 7.5 ft. In meters, f/1.4 × 26 m = 37 as do f/22 × 1.7 m 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", and "speedlights")[note 1] 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:[1]

Guide number = f-number × distance

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.[2] Accordingly, as illustrated at right, a guide number can be factored to a small f‑number times a long distance just as readily as a large f‑number times a short distance.

Guide numbers are not an absolute measure of flash power (a property called luminous energy) because they are influenced by other variables, notably the camera's ISO setting (film speed) and flash coverage angle.[note 2] For any given ISO setting and coverage angle however, an inherently more powerful flash device featuring a guide number that is, for instance, twice as great will permit subjects to be properly exposed from twice as far away for any given f‑stop, or conversely, will permit scenes at a given distance to be properly exposed at 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.[3] 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.[note 3] 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.[4] However, manufacturers sometimes rate guide numbers at ISO 200, which makes them 41% greater.[5] Furthermore, the illumination angles underlying manufacturers' ratings vary greatly, which can make it particularly difficult to compare models.


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  1. ^ B & H Foto & Electronics: Understanding Guide Numbers
  2. ^ Scantips.com: Understanding Flash Guide Numbers, plus GN Calculator
  3. ^ Jacobson, Ralph (2000). Manual of Photography (9th ed.). Focal Press. p. 331. ISBN 978-0-240-51574-8.
  4. ^ One notable way of expressing guide numbers scaled for use with both feet and meters (and relative to an ISO setting of 100) is as practiced by Metz mecatech GmbH, as exemplified by the data sheet for their mecablitz 52 AF-1 digital (product page here):
    Flash output
    • High max. guide number 52 (meters), High max. guide
    number 170 (feet) for ISO 100/21° and 105 mm
    Another notable example is Nikon's practice for expressing the technical specifications of products marketed in the U.S., such as their SB-5000 AF Speedlight (product page here):
    Guide Number
    34.5 m/113 ft. (at 35 mm) 55
    m/180 ft. (at 200 mm) (FX
    format, standard illumination
    pattern) (at ISO 100)
    Yet another notable example is Canon USA's practices, as exemplified by their Speedlite 430EX III-RT (product page here), which is as follows:
    Guide Number  The maximum Guide No. is approximately 141.1 ft./43m at ISO 100 and 105 mm flash coverage.
  5. ^ Nikon, for instance, gives two ratings when providing the technical specifications for their SB-910 AF Speedlight, one of which is relative to ISO 200 (product page here):
    Guide Number
    34 m/111.5 ft. (at ISO 100,
    35mm zoom head position, in
    FX format, standard
    illumination pattern,
    20°C/68°F) to 48 m/157.5 ft.
    (at ISO 200, 35 mm zoom
    head position, in FX format,
    standard illumination pattern,
    20°C/68°F)