Flash Metering System of the Canon EOS Digital Camera.
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.
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 shutter speed of a camera will only be effective if the light from the environment is a continuous source. However, flash light burst is so brief that the camera shutter hardly has time to register the change in the amount of light. As such to control the amount of light of a flash unit entering the camera;
Change the physical diaphragm of the camera lens which is the lens aperture. Doing so allows you to control the amount of light that can enter the camera.
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.
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.
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 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.
Discussion Area - Leave a Comment
You must be logged in to post a comment.