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Thread: aperture diameter

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    Default aperture diameter

    If the aperture diameter is given by focal length/f-number, won't a 105mm at f4 have roughly the same aperture diameter as a 50mm at f1.8? Does the aperture diameter give the size of the aperture such both lenses will be as fast as each other at that ratio?

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    Moderator ziploc's Avatar
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    Default Re: aperture diameter

    Quote Originally Posted by canalrat View Post
    If the aperture diameter is given by focal length/f-number, won't a 105mm at f4 have roughly the same aperture diameter as a 50mm at f1.8?
    Yes roughly.

    Quote Originally Posted by canalrat View Post
    Does the aperture diameter give the size of the aperture such both lenses will be as fast as each other at that ratio?
    No. The image that is projected onto the film plane decreases in brightness when the focal length gets longer due to optical magnification. In order to compensate for the lost of light due to longer focal length, the pupil size of the aperture (it's area) needs to get larger. That is why f-stops are expressed in terms of the ratio of focal length against pupil diameter. So f/4 at 105mm gives the same exposure value (EV) as f/4 at 50mm, and f/4 at 105mm does not give the same EV as f/1.8 at 50mm.

    In other words, at f/4, 50mm : pupil diameter = 50mm/4 = 12.5mm. In order for a 105mm lens to have the same EV, pupil diameter now needs to be larger = 105/4 = 26.25mm. This is the exact reason why long lenses need to be bigger comparing to standard lenses at the same aperture (e.g. 300mm vs 50mm at f/2.8).
    Last edited by ziploc; 27th May 2010 at 04:39 PM. Reason: clarify

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