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 [业余八卦探长] 小章的照片在多远拍摄的？欢迎数学物理大师们修正。 |
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| 所跟贴 |
IMHO, since image is sharp, the F# is big -- theoretical - (255 Byte) 2009-1-08 周四, 10:08 (369 reads) |
土匪
头衔: 海归少尉
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加入时间: 2009/01/04
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作者：土匪 在 海归酒吧 发贴, 来自【海归网】 http://www.haiguinet.com
If the F# is big, which means the numerical aperture (NA) of the lens is small, then the depth of the image is large. This is our case, so you are not only clearly taking photos of Zhang Ziyi, but also the guy with the fin as well.
If the F# is small, which means the numerical aperture of the lens is big, then the depth of the image is small, so you can only take photos of Zhang Ziyi clearly, the environment will be blurring.
So what is the F#? It equals to the focal length of lens over the diameter of the entrance pupil of the lens. It has nothing to do with the shutter speed. When I say the shutter speed is 1/125s, it means the exposure time is 1/125 second. It is nothing related to the focal length of the lens.
Super telephoto lenses usually have the focal length over 300mm-800mm, or even longer. Please check the Canon website. And they usually have the smallest F# of 4 or 5.6. But when you are taking the photographs, you can easily increase the F# of these lenses by making the diameter of the entrance pupil of lens smaller. How can you do it? Just make the stop aperture before the lens smaller. This is what the AV mode of a SLR does!
So the super telephoto lenses can be used to take the images with large depth, which is the current case and the background is not blurring.
Do I make the physics clear??  )
作者：土匪 在 海归酒吧 发贴, 来自【海归网】 http://www.haiguinet.com
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