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Old Nov 24, 2004, 2:04 AM   #1
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If the F stop is a function of lense diameter/focal length ratio...and the focal length is dependent upon the radius ground into the lense, how can they create lenses with multiple f stops? And another thing...how do Panasonic and Nikon get so much zoom from such a short focal length? Do they "water down" the optical image in the same way digital zooms spread pixels? How are lenses tested foraccuracy? With light wave length? Which color? Yellow green? And to what fraction of a wave? Or do they do it all mathematically? Hmmmm....best regards,

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Old Nov 24, 2004, 5:29 AM   #2
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The f-stop quoted for a lens (as in 50mm f/1.8) is the MAXIMUM aperture size, and is the ratio of the aperture diameter to the focal length. The lens has an adjustable aperture inside, though, situated at a point where it doesn't produce an image--it just chokes down the amount of light getting through.

High-ratio zooms (like the popular "10x" zooms most digital camera makers have now) use several internal lens elements which move to magnify the image. The "short focal length" comes from the fact that a digital camera's sensor is quite a bit smaller than a 35mm film frame, so if you imagine putting a short-focal-length lens on a 35mm camera and cropping the picture that results, you can see how a short focal length lens has a longer "effective" or "35mm-equivalent" focal length on a digital camera. The image isn't "watered down" or anything; it's just being formed on a smaller area within the camera. (You could theoretically have a 38-380 f/3.5 zoom for a 35mm SLR, but it would be rather large and heavy. I know my 80-200 f/3.5for my Canon AE-1 is quite a chunk. Since my Minolta DiMage Z1 has a sensor that's a small fraction of the 24x36mm film frame's area, the lens can be a lot smaller and lighter to give the equivalent view.)
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Old Nov 24, 2004, 5:42 AM   #3
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FYI: http://digilander.libero.it/fotorepo...ca/sk_05_e.htm

KENNETHD wrote:
Quote:
If the F stop is a function of lense diameter/focal length ratio...and the focal length is dependent upon the radius ground into the lense, how can they create lenses with multiple f stops?
The different f-stop comes from the fact that this ratio changes when the focal lenght is varied by the zoom action (while the diameter of the lens physically staying constant)



Quote:
... how do Panasonic and Nikon get so much zoom from such a short focal length?
The same way a 1.4x or 2x teleconverters does it -> A focal lenght can be magnify greatly without increasing the lenght of the actual lens! :shock:



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How are lenses tested foraccuracy? With light wave length? Which color? Yellow green? And to what fraction of a wave? Or do they do it all mathematically?
See here: http://www.graphics.cornell.edu/~wes...res-chart.html :-) :-) :-)
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