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aperture/Fstop question

NaftoliNaftoli Member
edited April 2013 in home
it is my understanding that aperture size is determined by focal length divided by Fstop, which means for example a 70-200 f/2.8 lens set at 2.8 will have a larger diaphragm/aperture at 200mm than at 70mm and allow in the same amount of light, this can explain why a typical kit lens has a variable aperture of 3.5-5.6 b/c when zoomed in the aperture cant expand larger to accumulate equal amount of light from its zoomed out position.
If this is so than why isnt a 70-200 f/2.8 a 1.8 - 2.8 lens since at 70mm its aperture is not its widest at 2.8?
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