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Old Jan 23, 2003, 5:23 PM   #1
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Default Aperture Values

Can somebody answer a question that has bugged me for years.

Why does a small aperture value equate to a wide aperture while a large value equate to a small aperture.

Thanx

Tim
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Old Jan 23, 2003, 5:55 PM   #2
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Well, I don't know the answer to that, guess it falls under the same question as wire sizes.
#10 wire is thicker than #18 wire, as the numbers go up the wire size goes down.
Hey...I think we both need an answer to that one.
Bill
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Old Jan 23, 2003, 6:53 PM   #3
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Great Question: I know it has to do with the logrithmic reaction between light and film sensitivity, ie ISO/ASA.

The early lenses were marked in "Universal" F stops and were marked just the opposite. I have one on an antique. These new values were not introduced until the mass production of film/image material to get something into a stable mode.
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Old Jan 23, 2003, 7:11 PM   #4
r-w
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Default aperture

The aperture or "f-stop" is the ratio between the size of the aperture and the focal length of the lens. This value is calculated by taking the focal length of the lens and dividing it by the actual aperature in mm. A standard 50mm lens with an aperature of 25mm would have an f-stop value of 2. The smaller the aperature
increases the ratio and as such decreases the amount of light reaching the film surface... or in your case the ccd. This would be a good way to teach our kids fractions as well as developing a cool hobby...what do ya think?
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Old Jan 23, 2003, 7:33 PM   #5
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Default Re: aperture

Quote:
Originally Posted by r-w
... This would be a good way to teach our kids fractions as well as developing a cool hobby...what do ya think?
Sounds good. Can also use it to teach them about area vs linear measurement. The f/stop value is a dimensionless linear measurement while the amount of light that an aperture admits depends on its area, piRsquared and all that. It is the square that means to double the amount of light coming through the aperture, the radius (f/stop) needs to be increased by the square root of two, e.g., going from f/2 to f/2.8 doubles the amount of light so the shutter speed can be cut in half.
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