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# 4/3 has no focal length advantage-correct him if he is wrong

Started Nov 15, 2008 | Discussions thread
Re: 4/3 has no focal length advantage-correct him if he is wrong

Look, the focal length of any lens for any camera is shown as the distance from the "nodal point" of the lens to the film/sensor. Thus f8.0 for any lens means that the diameter of the aperture of that lens is 1/8th the focal length. It will allow as much light on the sensor as f8 does on any other size sensor (assuming all lenses are clean etc.)

For a 4/3 sensor a 25 mm lens at f8 means that the aperture is 25 divided by 8. The fact that that is smaller than f8 on a 200 mm lens makes no difference to the exposure, as the total amount of light has to travel eight times as far on the longer lens to reach the sensor. A schoolboy's knowledge of the inverse square law can show that an aperture of 3.125 mm (f8/ 25 mm lens) allows the same amount of light to arrive at the sensor as that from a 200 mm lens whose aperture is open at a diameter of 25 mm.

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