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11/27/2005 10:45:15 PM · #1 |
I thought I had f-stops figured out as the ratio of the opening but if this was true then how would the f-stop go below 1 as in this Canon f0.95.
So what am I missing? |
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11/27/2005 10:48:57 PM · #2 |
Originally posted by robs:
I thought I had f-stops figured out as the ratio of the opening but if this was true then how would the f-stop go below 1 as in this Canon f0.95.
So what am I missing? |
You're not missing anything, if the lens opening is larger than the focal length, say 50mm focal length and 52.6mm aperture, then the f-ratio is 50/52.6 = 0.95
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11/27/2005 10:51:06 PM · #3 |
| I know it's probably been mentioned here a million times but what is the formula to work out f-stops for your focal length and shutter speed? |
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11/27/2005 10:53:38 PM · #4 |
To be precise, f/stop is the ratio of the physical diameter of the aperture to the focal length of the lens. A 25mm aperture on a 50mm lens is f/2.0. The same physical perture (25mm) on a 100mm lens would be f/4.0. As Kirbic points out, f/0.95 designates a lens with a maximum aperture slightly larger than its focal length. Rare, but not uneard of.
R. |
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11/27/2005 10:56:27 PM · #5 |
Originally posted by kirbic:
Originally posted by robs:
I thought I had f-stops figured out as the ratio of the opening but if this was true then how would the f-stop go below 1 as in this Canon f0.95.
So what am I missing? |
You're not missing anything, if the lens opening is larger than the focal length, say 50mm focal length and 52.6mm aperture, then the f-ratio is 50/52.6 = 0.95 |
OIC, The penny drops so to speak.
So that would explain why they are not common cause the thing would get huge very quickly. |
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11/27/2005 10:57:20 PM · #6 |
Originally posted by Makka:
I know it's probably been mentioned here a million times but what is the formula to work out f-stops for your focal length and shutter speed? |
You mean, what's the relationship between them? Opening up one full stop is the same as doubling the light coming in, so shutter speed is doubled. Close down one stop, say from f/4.0 to f/5.6, and you halve the light, so halve the shutter speed. The f-stop is defined as the focal length divided by the aperture size, so as focal length is increased, the aperture size must increase to keep the f-stop constant. That's why long, fast lenses have such large front elements. It's also why f/4 is f/4, no matter the focal length. The amount of light on the sensor is the same, for the same scene.
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11/27/2005 11:00:34 PM · #7 |
Originally posted by robs:
Originally posted by kirbic:
Originally posted by robs:
I thought I had f-stops figured out as the ratio of the opening but if this was true then how would the f-stop go below 1 as in this Canon f0.95.
So what am I missing? |
You're not missing anything, if the lens opening is larger than the focal length, say 50mm focal length and 52.6mm aperture, then the f-ratio is 50/52.6 = 0.95 |
OIC, The penny drops so to speak.
So that would explain why they are not common cause the thing would get huge very quickly. |
Yeah, real quick :-) A 100mm f/1.0 would have a 100mm aperture, requiring a barrel diameter of eoughly 5.5 inches to include all the machinery, at a minimum. That's big-telephoto weight-of-glass.
R. |
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11/27/2005 11:12:08 PM · #8 |
Originally posted by bear_music:
Yeah, real quick :-) A 100mm f/1.0 would have a 100mm aperture, requiring a barrel diameter of eoughly 5.5 inches to include all the machinery, at a minimum. That's big-telephoto weight-of-glass.
R. |
Do remember, though, that it's technically not the size of the actual diaphragm aperture, it's the "aperture" of the light-collecting front element. For instance, the 70-200/2.8 would require an effective aperture of 71.4mm, and has a 77mm filter thread... not a lot of room at all beyond the aperture that's theoretically required! The (usable) front element diameter is what counts.
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11/27/2005 11:13:30 PM · #9 |
Originally posted by kirbic:
Originally posted by Makka:
I know it's probably been mentioned here a million times but what is the formula to work out f-stops for your focal length and shutter speed? |
You mean, what's the relationship between them? Opening up one full stop is the same as doubling the light coming in, so shutter speed is doubled. Close down one stop, say from f/4.0 to f/5.6, and you halve the light, so halve the shutter speed. The f-stop is defined as the focal length divided by the aperture size, so as focal length is increased, the aperture size must increase to keep the f-stop constant. That's why long, fast lenses have such large front elements. It's also why f/4 is f/4, no matter the focal length. The amount of light on the sensor is the same, for the same scene. |
Cheers for that kirbic! :) |
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11/27/2005 11:13:56 PM · #10 |
| Thanks for the help guys - As they say, it's seldom the stuff you don't know but the stuff you do that isn't right (or something to that affect). |
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11/27/2005 11:19:50 PM · #11 |
| Kurbrick had several 50mm Zeiss lenses manafactured for NASA with a f-stop of .7 modified to use on their Mitchell BNC 35mm cameras for the film Barry Lyndon . The beautiful interior scenes were shot with candle-light as the only light source and that's with the slow film stock available in 1975. Focus had to be dead on due to almost nonexistant DOF. |
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11/28/2005 04:12:08 AM · #12 |
I get stuck on the english terms. Can anyone translate it to dutch for me ? :-(
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