What is the flash index?

Lead: Flash is one of the necessary equipment for photography. Do you know how to use the flash and related parameters? Do you know what the flash index is? Come and have a look with me!

What is the flash index? The flash index gn is one of the indexes reflecting the flash power. A good flashlight should have stable and adjustable output, standard color temperature (generally around 5500k, the same as sunlight), fast return speed, turnability and variable illumination range.

Flash index: GN. Flash index is related to the use of aperture, which is one of the indicators reflecting flash power. The flash index has two functions: one is to identify the flash power, and the greater the GN value, the greater the power; The other is to calculate and determine the aperture size of flash exposure when using manual flash photography. The basic formula is as follows

Flash index/distance = aperture coefficient

An important parameter to identify the performance of flash (now all refer to electronic flash) is the power of flash. The power of flash is usually exponential? GN？ Incidentally, for ISO 100 sensitivity film or digital camera settings, GN = aperture factor x shooting distance (m), because the flash light-emitting time is very short (1/300 seconds-1/20000 seconds), so as long as the shutter speed can synchronize the flash, it has little influence on the flash exposure, and the exposure is determined by the aperture and flash. For example, if the flash with index GN =24 is used as the main light, the sensitivity is set to ISO 100, and the shooting distance is 3 meters, then the reference aperture coefficient for correct exposure is 24/3 =F8. If you use film with other sensitivity or adjust the sensitivity of a digital camera, you can use the following formula to calculate the new index:

New index = square root (new ISO value/100) x nominal index

Flash with automatic flash function can control the output ratio according to the reflection of the subject, such as E-TTL automatic flash and pre-flash before formal shooting. According to the reflection of the subject, not only the output of the flash can be changed, but also the exposure combination and even the white balance of the camera (digital camera) can be changed.

In the instruction manual of electronic flash, the manufacturer indicates the flash index of the flash for you (commonly used? GN？ Say). The flash index has two functions for you: first, it allows you to identify the power of the flash. GN？ The greater the value, the greater the power; Second, when you shoot with a manual flash, it is up to you to calculate and determine the aperture size of the flash exposure. The basic formula is:

Flash index? Distance = aperture coefficient

When using this basic formula, we should pay attention to the meaning of this calibrated flash index. In many cases, it is necessary to flexibly apply the flash index.