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you can use f2.8 throughout the whole zoom range but can also use any other aperture too.
the confusion comes with cheaper lenses which quote apertures of say 4.5-5.6, this means they can be used at f4.5 at the low end (eg 70mm) but only as low as 5.6 when zoomed in (eg at 300mm), they can of course use all apertures above 5.6 at all ranges.
the aperture quoted for a lens is always the widest aperture available, though there are some lenses which have a fixed aperture, for instance mirror lenses which only have f8 but these are quite rare now.
Last edited by Cathus; 04-11-11 at 04:27 PM.
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