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If you look at the lens marking, you see how most lenses list a range of apertures along with the focal length... like my Canon EF-S 17-85mm, also lists "1:3.5-5.6" (same as "f/3.5-f/5.6"). That tells you the maximum aperture at each end of the zoom - at 17mm I can get up to f/3.5; at 85mm it's only f/5.6.
Higher-end zooms are often designed to maintain a constant maximum aperture through their whole length (at additional cost, of course), but the vast majority of zooms have this "limitation".
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