Guide number

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The guide number is the number used to indicate the light energy of a flash unit - the flash range.

Definition and calculation

Guide number

The guide number is the product of the distance between the flash and the subject in meters and the f-number required for correct exposure :

The guide number can be used to calculate the distance between the flash unit and the subject at which the flash can sufficiently expose an image, or, conversely, with a given guide number and distance, the f-number to be set objectively on the camera can be determined.

Relationship between the distance of the object, the aperture set on the lens and the guide number of the flash unit

This specification often refers to a film speed of ISO 100/21 ° (with historical flash units partly up to the 1970s ISO 50/18 °) and an aperture of 1: 1, but this specification is neither standardized nor is it being used they are consistently applied by all manufacturers of flash units. It is therefore usually necessary to convert the guide number given by the manufacturer into the actual guide number that corresponds to your own equipment and film speed. The effective guide number also depends on other factors, such as zoom reflector setting and manual power setting on the flash unit as well as the reflective properties of the object and the surroundings. Most manufacturers give the guide number for a relatively small interior space with white walls, which increases the light output due to the light reflected from the walls.

The brightness of the illuminated object decreases with the square of the flash distance. A flash unit with double the guide number therefore delivers four times the amount of light.

The maximum distance of the object , the maximum flash range results from the guide number of the flash unit and the smallest possible aperture number of the camera:

If a camera is used in manual mode , the guide number can be used as a calculation aid for the f-number .

Modern flash units ( computer flash ) measure the light reflected from the subject through a sensor and thus control the duration of the flash. In conjunction with cameras equipped for this purpose, the amount of light can also be measured and controlled through the lens ( Through The Lens , TTL). Some simpler digital cameras also set the light output of the built-in flash by means of a guide number calculation by using the set aperture and the object distance determined by the autofocus .

Other specifications for the amount of light from a flash unit , mainly used in studio flash systems, are watt seconds , and more rarely BCPS (Beam Candle Power Seconds) or ECPS (Effective Candle Power Seconds) . Physically, the amount of light has the unit lumen second .

Lightning range

In flash photography, the flash range is the distance over which a flash unit can sufficiently illuminate the subject to be photographed. The flash range depends on the light output of the flash unit, indicated by the guide number, the set f-number of the camera lens, the film sensitivity and the reflective properties of the surroundings and the subject.

The following formulas apply to a "medium brightness" motif (18% gray, see gray card ).

The flash range (A) is calculated from the guide number (L) of the flash unit divided by the set aperture (B) of the camera lens:

Example : If the flash has the guide number 12 (corresponds to many internal camera flashes) and the aperture is set to 5.6, the maximum range is 2.14 m:

The formula is only valid in this form if the film speed corresponds to the speed to which the guide number indicated on the flash unit relates. The guide number often refers to a film speed of ISO 100/21 °.

The flash range can be increased by using a film with a higher light sensitivity. The range changes by the square root of the ratio of the actual film speed (E F ) to the film speed (E L ), to which the guide number of the flash unit refers. This results in a factor (F) by which the flash range must be multiplied:

If the guide number refers to a film speed of ISO 100/21 °, the following conversion factors result for the most common film speeds:

Movie factor
ISO 50 0.7
ISO 100 1.0
ISO 200 1.4
ISO 400 2.0
ISO 800 2.8
ISO 1600 4.0
ISO 3200 5.7

Quadrupling the film speed doubles the flash range.

Taking into account the film speed, the following formula is obtained for the flash range:

Example : If the guide number is set again to 12 and the aperture is set to 5.6, the film speed is ISO 800 and the guide number relates to a film speed of ISO 100, the flash will reach 6.06 m:

Adjustable reflector

Many flash units offer the option of using an adjustable reflector to adapt the illuminated image angle to the image angle of the camera. Usually, the focal length of a lens is specified that has a suitable angle of view on a 35mm camera . The guide number changes with the size of the illuminated area.

Example (Nikon SB 800):

Angle of view 114 ° 104 ° 84 ° 75 ° 63 ° 47 ° 34 ° 29 ° 23 °
Focal length (35mm) 14 mm 17 mm 24 mm 28 mm 35 mm 50 mm 70 mm 85 mm 105 mm
Guide number (ISO 100, meters) 17th 19th 30th 32 38 44 50 53 56

See also

Portal: Photography  - Overview of Wikipedia content on photography

literature

  • Andreas Feininger: Andreas Feininger's great photo apprenticeship . Düsseldorf, Vienna, 1979, ISBN 3-453-17975-7 .
  • ISO 1230: 2007-09, Photography - Determination of guide numbers for electronic flash units

Web links