The modern Electronic flash was invented by Harold Edgerton in 1931. Its introduction into the modern photographic world changed the photography as we know it today. The theory behind flash photography is that when you take a capacitor or condenser, charge it up and then release the electrical energy through a glass bulb filled with inert gases (Flash Light Bulb), you will get a split second burst of bright light. This emission of light occurs immediately after you send a pulse of electrical energy to the flash bulb. As such, there is a mean of control of the light output by way of controlling the electrical pulse duration to be sent to the bulb. Previously, a component called the thvristor was used to control this duration of electrical pulse. Nowadays, this process is automated with the use of electronics.
In normal photography, there are two ways to control the amount of light entering camera. One way is by adjusting the shutter speed which affects HOW LONG the exposure time will be. The other way is by adjusting the aperture of the lens. This will affect HOW MUCH light will enter the camera. You can also use filter to block out additional light but in this guide we will only discuss about the basic ways how flash exposure can be controlled. Flash photography differs from normal photography because flash timing involves split second of light. As such, the shutter speed does not play any role in determining the amount of light entering a camera except when the camera is in FP mode.
The speed of the camera’s shutter is only important when the ambient light is a continuous source. But the duration of a flash pulse is too brief for the camera mechanical shutter to even react to the change of light. Therefore the way to control the quantity of a flash pulse entering the camera is by:
You can change the size of the aperture of the lens. This is the physical diaphragm of the lens which controls the amount of light entering 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.
One can also control how much light there is between the flash unit and the subject matter by using diffuser. Although this is a possible way, one also has to consider the inconveniences of carrying all these items around on location.
Finally, by regulating the duration of the electrical pulse to the flash unit, you can control the duration of a flash burst. Therefore you will change the final flash output by such control. In fact, this is the primary way nowadays people use to control the electronic flash.
In short, this is the basic principle of flash metering. To have exact adjustment is difficult and requires many years of experiences before a person can correctly judge the right length of time to send power to the flash unit. Luckily for us, nowadays modern Camera like the Canon EOS digital camera has a system which automatically calculates the duration and control the flash pulse for the ideal picture.