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:B & H Foto & Electronics: Understanding Guide Numbers
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.Scantips.com: Understanding Flash Guide Numbers, plus GN Calculator 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.For example, among hot shoe-mounted flash devices, at ISO 100, 60° horizontal coverage angle (suitable for use with a 35 mm-format, full-frame lens with a focal length), and full power, the Holga Holgon MF Flash has a guide number of . Under the same conditions, both the Sony HVL-F58AM and the Metz mecablitz 64 AF-1 digital have guide numbers of . And still under the same conditions, the side-mounted pistol grip-style Metz mecablitz 76 MZ-5 digital is rated at . 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. mecablitz 52 AF-1 digital data page - Metz mecatech GmbH SB-5000 AF Speedlight product page - Nikon Speedlite 430EX III-RT product page - Canon However, manufacturers sometimes rate guide numbers at ISO 200, which makes them 41% greater.SB-910 AF Speedlight product page - Nikon): The illumination angles underlying manufacturers' ratings vary greatly, which can make it particularly difficult to compare models.
Understanding guide numbers Units of measure
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): .mecablitz 76 MZ-5 digital product page specifications - Metz mecatecheh GmbH 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: . 'VS 510P product page - Bolt B & H Foto & Electronics on-camera flash page 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: / .
Though nomenclature conventions such as these can make guide numbers misleadingly appear to be units of measure, they serve as to eliminate ambiguity as to which length-based system of measurement underlies the guide numbers.
Calculating with guide numbers 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 / = .
Details
The magnitude of guide numbers is a function of the following four variables: The total luminous energy (in lumen⋅seconds) emitted by the flash head (which is itself the product of the duration and the average luminous flux of a flash). The solid angle subtended by the circular- or rectangular-profile beam as it leaves the flash head (the average of the beam's horizontal and vertical angles). The ISO sensitivity setting. Filters (either on the flash or on the camera lens). .
The above variables fall into two classes that influence the magnitude of guide numbers:
Those that affect the non-distance-related intensity of a flash arriving at a scene (a property called illuminance, measured in lux or lumens per square meter) or its duration; namely its power setting, flash coverage angle, and color gels in front of the flash head. Those that affect the camera's non-aperture-related light sensitivity; namely lens filters and the ISO rating of the film/imaging sensor.
Changing either the f-stop or the flash-to-subject distance does affect guide numbers because, by definition, choosing a different value for one factor is automatically accompanied by a reciprocal adjustment in the other.
Most modern flash devices can operate at guide numbers less than their maximum ratings via either manual adjustment of their power settings in binned steps, by an in-flash automatic light sensing feature, or by being cued by the camera's sensor; both the latter options make continuously variable adjustments. Manual attenuation settings are usually steps in powers of 0.5 (full f-stops) that commonly extend five to eight f-stops deep (power levels of , , ... ).The Canon Speedlite 430EX III-RT goes down to 1/128; the Nikon SB-5000 AF Speedlight goes down to 1/256. 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. Whereas reflectance (the albedo of the objects being photographed) have no influence on the illuminance arriving at any given scene, reflective surfaces between a flash device and a scene, such as light-colored ceilings, do have an influence. Guide numbers are often calculated by manufacturers for optimum results in average indoor settings.Characterizing the Output of Photographic Flash Units, by Douglas A. Kerr (PDF here) for more. 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.
Effect of power settings Most modern electronic flash devices have manually adjustable power settings. Moreover, virtually all modern on-camera flash devices that have manually adjustable power settings also provide either a built-in mechanical circular calculator (such as shown in the photo at the top of this article) or a digital display that automatically shows the effect power levels have on f-stop and distance (guide number).
Nevertheless, for those who want to master the math, guide numbers diminish from their full-power ratings as the square root of their fractional setting per the following formula:
For example, suppose a flash's guide number is . At power, the reduced-power GN is .
Effect of flash angle (zoom setting) Many flash devices have auto- or manual-adjust zoom features that permit the illumination angle to be widened (lessening the guide number) to fully illuminate the image area of wide-angle lenses, or narrowed (increasing the guide number) for telephoto lenses. Such coverage angles may be given in degrees but are often expressed as being equivalent to lens focal lengths for full-frame, 35 mm cameras. Manufacturer advertising practices vary as to the angle of coverage underlying their guide number ratings, in large part because some flash devices can be zoomed whereas others are fixed.
Virtually all modern on-camera flash devices with zoomable flash heads also have either a built-in mechanical circular calculator (such as shown in the photo at the top of this article) or a digital display; both automatically show the effect zoom levels have on f-stop and distance (guide number).
The optics of flash heads are complex; each manufacture's designs not only have illumination areas that are slightly different, but are the product of differing relative proportions of transmission, diffusion, reflection, and refraction among their optical elements (flash tube, reflector, Fresnel lens, and add-on wide-angle adapter). Accordingly, there is no universal formula for precisely calculating how guide numbers diminish from, for instance, a setting to or settings.