What does ... mean?

A: The flash index (GN) is a measure of the ability of a flash to illuminate a target with a certain sensitivity and viewing angle. The flash lamp with high flash index has higher irradiation power.

For example, if the flash index becomes k times, the power of the flash will become k 2 times, so that the flash can illuminate the target object at a distance k times longer than the original distance to achieve the same lighting effect; Or shoot the target object at the same position with the sensitivity of1/k 2.

Extension:

Sensitivity, also known as ISO value, is a measure of the sensitivity of negative film to light, which is determined by sensitivity measurement and several values measurement. The ISO standard is ISO 6. Films that are less sensitive to light need to be exposed for a longer time to achieve the same imaging effect as photosensitive films, so they are usually called slow films.

Highly sensitive negatives are therefore called fast negatives. ISO system closely related to sensitivity is used to measure the sensitivity of digital imaging system. Whether it is digital or negative photography, using higher sensitivity to reduce exposure time usually leads to the decline of image quality (due to thicker negative particles or higher image noise or other factors).

Basically, the higher the sensitivity, the worse the photo quality.

Baidu encyclopedia -Flash index