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Old Mar 29, 2006, 7:36 PM   #1
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Are aperture characteristics supposed to be the same from lens to lens? That is, is aperture an absolute measurement: f5.6 on a DA 18-55mm = f5.6 on a FA 50mm ?

I'm thinking this is so, as aperture is literally an aperture (which can be standardized) through which the light enters the lens, but wondered if an aperature value is somewhat relative from lens to lens.
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Old Mar 29, 2006, 7:52 PM   #2
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yes,

aperture is a measurement of the light hitting the negative,sensor. the same amount of light is hitting the film plane when using a 500mm, f8 as is when using a 28mm, f8. if it wasn't a constant, there would be no way to calculate exposure.

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Old Mar 29, 2006, 9:41 PM   #3
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hi,

this is how i understand it:

aperture varies in actual size from one focal length to another because the number given to each aperture is a ratio of the diameter of the chosen aperture compared to the length of the lens. however theshutter speedremains the same between two different lenses at the same aperture for the same exposure value (although the meters in different camerasmay suggestslightly different settings).

in other words the size of the opening at f/4 on a 100mm lens will be larger than f/4 on a 50mm lens, but the amount of light reaching the sensor or film will be the same.

i've always supposed this is because more light falls off inside the longer lens than in the shorter lens (similar to the way the light from a flash falls off with more distance) but given the state of my knowledge of the physics of light this may be either wrong or only one of several reasons.

i have read somewhere that the reason p&s's with their small sensors (and correspondingly short lenses) don't have smaller apertures than f/8 or f/11 is becauseof problems with the physical properties of light having to go through such small openings. (i think they spoke of the light "bending" around the edges of the aperture blades?) i also guess that's why those cameras also have several stopsmore DOF at a given aperture than the larger SLRs.

hope this helps , eric
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Old Mar 29, 2006, 10:40 PM   #4
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pwithem wrote:
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Are aperture characteristics supposed to be the same from lens to lens? That is, is aperture an absolute measurement: f5.6 on a DA 18-55mm = f5.6 on a FA 50mm ?

I'm thinking this is so, as aperture is literally an aperture (which can be standardized) through which the light enters the lens, but wondered if an aperature value is somewhat relative from lens to lens.
Yes, in terms of light energy/power reaching the film/sensor. No, in terms of (the size of) physical opening of the aperture, by definition, the diameter of the aperture is calculated by the focal length to be divided by the f-number.

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