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Old Mar 17, 2011, 8:58 AM   #11
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What the f/number is most useful for is determining the light gathering ability of the lens, and the resultant exposure. F/4 will give you the same amount of light at the sensor, regardless of the focal length of the lens. (under the same lighting conditions, of course) This very much simplifies exposure determination.
The smallest f/number for a lens will only give you a minimum value for lens objective diameter, since the numbers are standardized to give the same exposure value. The number of lens elements, and light transmission of them, will need to be taken into account.

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Old Mar 17, 2011, 10:09 AM   #12
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Ok, it seems obvious now. And now I see how it really isn't important. The main reason I wanted to know is for when explaining the variables of exposure (shutter speed, ISO, and aperture) to people; I knew what the numbers of ISO and shutter speed represent, but not the aperture. Thanks again!
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Old Mar 17, 2011, 12:50 PM   #13
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Since the angle of view varies with the focal length, so does the ability to gather light. A 100mmm lens has (roughly) half the angle of view of a 50mm lens, so it must have an aperture that is twice the diameter as that 50mm lens in order to gather the same amount of light, in order to produce the same exposure value at the same ISO and shutter speed.

A 100mm lens must have an aperture diameter of 50mm in order to gather as much light as a 50mm lens with an aperture diameter of only 25mm. In both instances, they would have an F-Number of f/2.0, and so would gather the same amount of light, and allow the use of the same ISO and shutter speed.
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