You'll see the pair of numbers on a zoom lens. Aperture is basically a ratio of the focal length to high wide the hole is. So when you are at the longer end of a zoom lens, the actual hole for the aperture isn't a different size but its relationship to the focal length is.
So in a nutshell, at longer zooms the amount of light that hits your sensor decreases.
Message edited by author 2009-06-22 17:30:21.
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