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<blockquote data-quote="WayneF" data-source="post: 396210" data-attributes="member: 12496"><p>Only if you open it wider to use it (potential, not actual in all cases).</p><p></p><p>f/stop number = focal length / aperture diameter (effective diameter, viewed through magnification of front elements).</p><p></p><p>The sole purpose of inventing f/stop is that f/8 is f/8 in any lens. A 200 mm lens will have an aperture area 4x larger than a 50 mm lens, so that f/8 will be f/8 in both. A faster lens may open wider, say to f/1.4, but when at f/8, its diameter is stopped down, so that is NOT f/1.4 then, it is a f/8 lens.</p><p></p><p>We could discuss minor variations, T-stops and transmission of lenses with poor coatings, or 15 lens elements, but the the big idea is that f/4 is f/4.</p></blockquote><p></p>
[QUOTE="WayneF, post: 396210, member: 12496"] Only if you open it wider to use it (potential, not actual in all cases). f/stop number = focal length / aperture diameter (effective diameter, viewed through magnification of front elements). The sole purpose of inventing f/stop is that f/8 is f/8 in any lens. A 200 mm lens will have an aperture area 4x larger than a 50 mm lens, so that f/8 will be f/8 in both. A faster lens may open wider, say to f/1.4, but when at f/8, its diameter is stopped down, so that is NOT f/1.4 then, it is a f/8 lens. We could discuss minor variations, T-stops and transmission of lenses with poor coatings, or 15 lens elements, but the the big idea is that f/4 is f/4. [/QUOTE]
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