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 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:
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.
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.
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.
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 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:
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.
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.
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
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