What is the flash index? . . Seek a detailed explanation

This indicator is only a nominal test data in a standard environment. When the situation changes, the index itself will also change. If the ISO is not 100, then the actual ISO divided by 100 and multiplied by the index marked by the flash is the actual index used to calculate the aperture under this ISO. For example, under ISO400, the index of BY-26AZ is 400/ 100=4, and then multiply the root by 26, and the answer is 52. At this time, if the distance is 10 meter, you only need to drive F5.0 as for the focal length, not the focal length of the camera, but the focal length of the zoom lamp holder of the flash. Output of the same power. If the angle of output light decreases, the light intensity per unit area will naturally increase. In addition, some people have been worried that the DC aperture is not small and the flash with large index will not explode at close range. In fact, BY-26AZ and other automatic flashlights have the ability to automatically adjust the output power, and nearby objects will automatically reduce the power output; Instead of automatic lights, it often has the function of power adjustment. Even for lamps with fixed power, you can extend the distance and get correct exposure by reflecting flash. But in case you choose a low-power flash, you can't play any reflective flash, diffuse flash. Therefore, it is right to choose a larger index within the economic scope.

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