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#61
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#62
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On Sat, 28 Aug 2004 06:07:06 GMT, Matt Ion wrote:
Bob wrote: The best lens we could have would be f 1. I hear NASA actually made one... any less then 1 is impossible since it would be inside out... If I remember rightly, one of the big selling points of the Canon EOS line when it first came out was that the lens mount was significantly larger than anything previously available on a 35mm SLR - large enough to accomodate the first f/1 50mm lens. BTW, lower f-stops are certainly possible: a 50mm f/.8 lens would simply be a lens with a 50mm focal length and a 62.5mm aperture (50/.8). I know - I was mistakenly thinking about the impossibility of putting one into a camera - not the impossibility of making one! BTW the last number on the lens doesn't count... a lens of 3.5 just happened to be that size, and that's the biggest it can be! It is not a full f stop away from the next standard number. (f4) That didn't make any sense at all... Sure it does... as you yourself not, the series of F-stops goes 11, 5.6, 4, and then 3.5... it should be 2.8.... but the lens is a 3.5... see? Commonly "marked" f-stops generally start at f/1 and go up by full stops (ie. 1/2 reductions in light). As you state, a full stop means a ratio of 1.4 in the diameter of the aperture, so we get full stops at f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/44, etc (every doubling or halving in aperture is two stops). The number listed on a lens is the maximum opening that lens is capable of, and it doesn't have to be on an exact stop (a 100mm, f/3.5 lens would have a max aperture of about 28.57mm). doesn't work that way... the standard series is always used... |
#63
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Bob wrote:
The best lens we could have would be f 1. I hear NASA actually made one... any less then 1 is impossible since it would be inside out... Guess again! Canon made a normal focal length lens with max aperture of f/0.95 back around the early 70's. The f/stop is simply the ratio of the focal length to the aperture's diameter. |
#64
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Bob wrote:
The best lens we could have would be f 1. I hear NASA actually made one... any less then 1 is impossible since it would be inside out... Guess again! Canon made a normal focal length lens with max aperture of f/0.95 back around the early 70's. The f/stop is simply the ratio of the focal length to the aperture's diameter. |
#65
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In article , Wilt W
writes Bob wrote: The best lens we could have would be f 1. I hear NASA actually made one... any less then 1 is impossible since it would be inside out... Guess again! Canon made a normal focal length lens with max aperture of f/0.95 back around the early 70's. The f/stop is simply the ratio of the focal length to the aperture's diameter. The theoretical maximum aperture of any lens in air, when focussed on a subject at infinity*, is f/0.5 (NA** = 1.0). In practice this is unattainable, f/0.7 is the practical limit, and only a few lenses*** have ever exceeded this limit (and not by much). However, lenses of f/0.75 or so are not uncommon in specialist applications. For a lens immersed in a medium having the same refractive index as the glass - as is often done for high-powered microscope lenses - the theoretical limit depends on the refractive index, but the practical limit is f/0.36 (NA = 1.4). I have two lenses of this aperture. * It is often overlooked that the f-number is dependent on the object/image conjugate distances; as you move the lens away from the film/sensor to focus on a subject closer than infinity, the aperture (as seen by the sensor, which is what matters) becomes smaller. **NA, or numerical aperture, is the designation used for the aperture microscope lenses and in certain other areas; in essence, f-number = 1/(2xNA). ***Outside the area of microscopy, where dry (i.e. non-immersion) lenses of NA 0.95 (f/0.53) are common. They are however a pain to use correctly, requiring cover glasses to be within 0.01mm of the design thickness. David -- David Littlewood |
#66
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British condoms.
-- Larry |
#67
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ROTFLOL!
Larry wrote: British condoms. -- Larry |
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