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.
On the whole there are two ways to control the amount of light which enter the camera. By adjusting the shutter speed you can determine how long the exposure time is. The other way is to adjust the aperture of the lens. By doing so you will how much light will get to enter the camera. However Flash photography is different from normal photography. This is because of the speed of a burst of the flash light. The time taken for a burst of light takes only micro seconds. This time frame is so short that the speed of a mechanical shutter is no longer a relevant factor in determining the amount of light entering a camera except when the camera is programmed under the 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.
One can by adjusting the physical distance between the flash unit and the subject matter, control the amount of light going into the camera. Light follows the laws of physics, so the further the distance one is from the light source, the lesser is the amount of light available.
Use a diffuser on the flash unit to reduce the amount of light emitted by the flash unit. The main draw back to this method is the inconveniences of having to carry all these items around.
Lastly, we can affect the duration of the flash burst by adjusting the duration of the electrical pulse to the flash unit therefore controlling the amount of light produced. This is the primary ways today how we control electronic flash.
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.
On the whole there are two ways to control the amount of light which enter the camera. By adjusting the shutter speed you can determine how long the exposure time is. The other way is to adjust the aperture of the lens. By doing so you will how much light will get to enter the camera. However Flash photography is different from normal photography. This is because of the speed of a burst of the flash light. The time taken for a burst of light takes only micro seconds. This time frame is so short that the speed of a mechanical shutter is no longer a relevant factor in determining the amount of light entering a camera except when the camera is programmed under the 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.
One can by adjusting the physical distance between the flash unit and the subject matter, control the amount of light going into the camera. Light follows the laws of physics, so the further the distance one is from the light source, the lesser is the amount of light available.
Use a diffuser on the flash unit to reduce the amount of light emitted by the flash unit. The main draw back to this method is the inconveniences of having to carry all these items around.
Lastly, we can affect the duration of the flash burst by adjusting the duration of the electrical pulse to the flash unit therefore controlling the amount of light produced. This is the primary ways today how we control electronic flash.
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.
About the Author:
Tim Harris is a canon camera enthusiast. For tips on choosing and buying cameras, do check out Canon OS Camera website. Camera 40D Digital Rebel XTI 3D Blog
