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Noob question - focal length vs aperture
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<blockquote data-quote="WayneF" data-source="post: 126481" data-attributes="member: 12496"><p>Right about the tubes, but wrong about aperture.</p><p></p><p>The overriding idea to know is that the design is so that f/2.8 always lets the same amount of light in, regardless of focal length.</p><p></p><p>The definition of f/stop is f/number = focal length / aperture diameter</p><p></p><p>This means at 70mm and f/2.8, the effective aperture diameter is 70mm / 2.8 = 25mm</p><p></p><p>At 200mm and f/2.8, the effective aperture diameter is 200mm / 2.8 = 71mm</p><p></p><p>So at 200mm, the tube might be 200/70 = 2.8 longer,</p><p>but the aperture is 71/25 = 2.8 wider, which exactly offsets length.</p><p></p><p>These numbers here are just approximations, because f/2.8 is technically and precisely 2.828, and the focal length varies slightly with focus distance. The marked focal length is true when focused at infinity.</p><p></p><p>The effective aperture is the diameter seen through the magnification of the front lens elements (same as the scene sees). Simply look into the front of this 70-200 mm lens as you rotate zoom from 70 to 200 mm, and you will see it grow much larger (to stay the same light transmission).</p></blockquote><p></p>
[QUOTE="WayneF, post: 126481, member: 12496"] Right about the tubes, but wrong about aperture. The overriding idea to know is that the design is so that f/2.8 always lets the same amount of light in, regardless of focal length. The definition of f/stop is f/number = focal length / aperture diameter This means at 70mm and f/2.8, the effective aperture diameter is 70mm / 2.8 = 25mm At 200mm and f/2.8, the effective aperture diameter is 200mm / 2.8 = 71mm So at 200mm, the tube might be 200/70 = 2.8 longer, but the aperture is 71/25 = 2.8 wider, which exactly offsets length. These numbers here are just approximations, because f/2.8 is technically and precisely 2.828, and the focal length varies slightly with focus distance. The marked focal length is true when focused at infinity. The effective aperture is the diameter seen through the magnification of the front lens elements (same as the scene sees). Simply look into the front of this 70-200 mm lens as you rotate zoom from 70 to 200 mm, and you will see it grow much larger (to stay the same light transmission). [/QUOTE]
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Noob question - focal length vs aperture
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