Principle of the Canon EOS digital Camera metering system
When Harold Edgerton invented the modern electronic flash in 1931, the world of photography was revolutionized. The principle behind the electronic flash is that when you charged up a capacitor or condenser and release the energy through a flash light bulb (A glass bulb containing inert gases), a split second burst of light will occur. This output of light occurs instantly when you send power to the flash light bulb. Therefore you can control this output of light through regulating the duration of electrical pulses to be sent to the flash light bulb. Before the advert of modern electronics, the duration of electrical pulses used to be controlled by a component called the thvristor. Today, modern electrical circuitry has taken over the function of this component.
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:
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.
Adjust the distance between the subject and flash unit. This method is an application of the laws of physics. The further an object is from a light source, the less illuminated the object is.
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.
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