What is the abbreviation of GN?

What is the guide number?

The guide index is an important indicator that expresses the performance of a flash and reflects the luminous intensity. It is represented by the English abbreviation "GN". According to international standards and our country's national standards, the definition of guide index is: when the film (or photosensitive element) in the camera obtains the appropriate exposure, the F-stop value of the camera lens times the distance L between the flash and the subject, It is called the guide index, that is, GN=F×L. Usually, the sensitivity is based on ISO100 and the distance is set in meters.

The guide index of the camera's built-in flash is very low, generally around GN.12, and the external flash can reach GN.30 or even above GN.50. Each flash has a different flash index. A safe distance for flash photography is about 2 to 4 meters, which can basically prevent flash photography from failing. Too close will result in overexposure, and too far away will result in underexposure. The effective distance of the flash is very limited. Due to the strong and dazzling brightness of the flash, many people think that it can reach a longer distance, but this is not the case.

Knowing the flash index, when taking flash photography, you can determine the aperture value of the lens according to the distance of the subject, or determine the effective distance of the flash according to the aperture value of the lens, that is, the flash can illuminate the Bright distance range. Since the flash flash time is very short, the shutter setting is not important at this time, just set it to a safety shutter.

The relationship between flash distance, aperture and sensitivity

Usually, in manual flash photography, the appropriate aperture is calculated based on the known guide index and photography distance. For example, It is known that the guide index of the flash is GN.24 and the shooting distance is 3 meters. According to the above formula, the appropriate aperture can be found to be F8 (24÷3). The greater the guide index of the flash, the smaller the aperture allowed at the same photographic distance, so that the subject can obtain a longer depth of field; similarly, at the same aperture, the effective photographic distance is the further away.

The relationship between sensitivity and flash distance follows the "flash distance rule", that is, multiply the distance by 2 and the sensitivity by 4. For example, assuming that the effective distance of the flash at ISO100 sensitivity is 5m, if you need the flash distance to reach 10 meters, you need to use a sensitivity of ISO400.