Guide number

As a guide number of the measure is called, at which the light energy of a flash device is specified - 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 respectively required for correct exposure f-number:

The distance between the flash unit and the subject over the guide number can be calculated, where the flash can illuminate an image sufficiently, or vice versa, for a given guide number and distance the aperture to be set on the camera lens can be determined.

This information often refers to a film speed of ISO 100/21 ° (for historical flash units partially into the 1970s on ISO 50/18 ° ) and an f-number of 1:1, but this requirement is not standardized, nor is they consistently applied by all manufacturers of flashguns. Therefore, it is usually necessary to convert the guide number specified by the manufacturer in the actual, of their own equipment and film sensitivity corresponding code. The effective guide number also depends on other factors, such as zoom-head setting and manual power settings on the flash unit and the reflection properties of the object and the environment. Most manufacturers specify the guide number for a relatively small interior room with white walls, in this case, the light output increases by the reflected light from the walls.

The brightness of the illuminated object decreases with the square of the flash distance. A flash with a guide number twice so provides four times the amount of light.

The maximum distance of the object, the maximum flash range is derived from the guide number of the flash unit and the smallest possible aperture number of the photographic apparatus:

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

Modern flash units ( computer flash) measure the light reflected from the subject by a sensor and so control the duration of the flash. In combination with specially equipped cameras, the amount of light can be measured and controlled by the lens ( Through The Lens TTL). Some simpler digital cameras provide the light output of the built-in flash also means guide number by the aperture setting and the determined from the auto-focus object distance can be used.

Others, mainly used in studio flash units to the amount of light of a flash are watt-seconds, more rarely, BCPS ( Beam Candle Power Seconds) or ECPS (Effective candlepower seconds ). The amount of light is physically the lumen unit second.

Flash range

As flash range is referred to in the flash photography, the distance over which a flash can illuminate the subject to be photographed adequately. The flash range is dependent on the light output of the flash unit, indicated by the guide number by the aperture of the camera lens, film speed and the reflection properties of the environment and of the subject.

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

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

Example: When the flash has a guide number of 12 (equivalent to many internal camera flash) is selected as the f/5.6, the maximum range is 2.14 m:

The formula is valid in this form only when the film speed matches the sensitivity, referred to by the specified the flash guide number. Often, the guide number refers to a film speed of ISO 100/21 °.

The flash range can be increased by a film with a higher sensitivity to light. The range varies by the square root of the ratio of the actual film speed (EF) of film speed ( EL), referred to by the guide number of the flash unit. This results in a factor (F), with the range of the flash must be multiplied:

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

A fourfold increase in film speed doubles the flash range.

Considering the film sensitivity is obtained for the range of the flash following formula:

For example, if the guide number is again set as 12 and f/5.6, the ISO sensitivity is 800 and the guide number refers to a film speed of ISO 100, the flash reaches 6.06 m wide:

Adjustable reflector

Many flash units offer the ability to customize means of an adjustable reflector, the illuminated field of view of the recording angle of the camera. Usually, the focal length of a lens is specified, which has a matching angle on a film camera. It varies with the size of the illuminated area, the guide number.

Example ( Nikon SB 800 ):