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Tokina 11-16mm.....Sigma 10-20mm f/stop question.
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<blockquote data-quote="Eyelight" data-source="post: 384579" data-attributes="member: 24753"><p>Not sure you really wanted to know the answer, and I mainly typed this out to get in straight in my head. But then figured why not share.</p><p></p><p>It is because we are doubling or halving the light and the number really means something related to doubling and halving. </p><p></p><p>At f/2 the aperture diameter is 1/2 the focal length of the lens. At f/4 it is 1/4 the focal length. The aperture at f/2 is twice the diameter as at f/4. </p><p></p><p><strong><span style="font-size: 15px">BUT,</span></strong> the area of the aperture at f/2 is FOUR TIMES the area at f/4. Four times the area allows four times the light, making 2 stops between f/2 and f/4. </p><p></p><p>The full stop in between is f/2.8, because an aperture diameter of 1/2.8 the focal length creates half the area for light to pass as the aperture diameter of 1/2 the focal length and twice the area than that of 1/4 the focal length.</p><p></p><p>So, f/2.8 is short hand for: Aperture diameter that is 1/2.8 the focal length of the lens.</p></blockquote><p></p>
[QUOTE="Eyelight, post: 384579, member: 24753"] Not sure you really wanted to know the answer, and I mainly typed this out to get in straight in my head. But then figured why not share. It is because we are doubling or halving the light and the number really means something related to doubling and halving. At f/2 the aperture diameter is 1/2 the focal length of the lens. At f/4 it is 1/4 the focal length. The aperture at f/2 is twice the diameter as at f/4. [B][SIZE=4]BUT,[/SIZE][/B] the area of the aperture at f/2 is FOUR TIMES the area at f/4. Four times the area allows four times the light, making 2 stops between f/2 and f/4. The full stop in between is f/2.8, because an aperture diameter of 1/2.8 the focal length creates half the area for light to pass as the aperture diameter of 1/2 the focal length and twice the area than that of 1/4 the focal length. So, f/2.8 is short hand for: Aperture diameter that is 1/2.8 the focal length of the lens. [/QUOTE]
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Tokina 11-16mm.....Sigma 10-20mm f/stop question.
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