Since the invention by Harold Edgerton in 1931 of the modern electronic flash, Photography has never been the same. The principle of electronic flash works by charging up a condenser or capacitor then releasing the charge in a split second burst of light from a flash light (a glass bulb filled with inert gases). The output of the flash light response instantly to the presence of power which is being sent to the flash light. As such the main form of control over the output of the light output is dependent upon the duration of electrical pulse sent to the bulb. It used to be that the electrical pulses was controlled by a component called the thyristor however today, modern flash units are completely automated by electronics.

Generally, there are two ways to control the quantity of light that enter the camera. The first way is adjusting the speed of the shutter which will affect how long the exposure time will be. The second way is through adjusting the aperture of the lens. This will determine how much light will enter the camera. Although you can use filter to block out additional light but here we will only talk about the fundamental ways of how to control flash exposure. Flash photography is an entirely different ball game because the flash pulse involves only micro seconds' display of light. As such, the mechanical shutter speed does not play any part in determining the amount of light which enter the camera except when the camera is in FP mode.

The Camera's shutter speed is only relevant when the ambient light is continuous source. Due to the fact that the flash pulse time span is so brief, the mechanical shutter have no time to even register any changes in the light source. Thus in order to control the amount of light which enter the camera, you have to:

Changing the lens aperture size. The aperture is the physical diaphragm of the camera lens and this control the quantity of light which enters the lens.

You can also adjust the physical distance from the flash unit to the subject. This will also control the quantity of light going into the camera. The laws of physics dictates that the further the distance between the light source and an object, the lesser the amount of light hitting the object.

You can also regulate the amount of light from a flash unit to a subject by using a diffuser. Of course, you have to consider the trouble of lugging around all these items on location.

By regulating the duration of electrical energy to be sent to the flash unit. By control the energy pulse, you control the duration of the flash burst. This allows you to determine how much light will be emitted to enter the camera. Nowadays, this is the principal way to control the electronic flash output.

In conclusion, this is the main principle of flash metering. The precise measure of flash metering is difficult and it requires years of experience before anyone can accurately judge the right duration for the electrical pulse to be sent to the flash unit. Nowadays, we are fortunate as camera like the Canon EOS digital camera fulfill this function automatically with modern electronics.