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Thread: constant f/stop zoom lens

  1. #1
    Raycer's Avatar
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    May 2008
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    constant f/stop zoom lens

    Just wondering how does constant aperture zoom lens work. Giving that the opening is inversely proportional to the focal length, does the opening actually close down when zoomed out?

  2. #2

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    Buenos Aires, Argentina
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    Re: constant f/stop lens

    I would say that it's because of the placement of the aperture in the lens and the movement of the elements as you zoom in or zoom out. The relation between diameter of the aperture and focal length -which should give a specific diameter of the diaphragm- is correct as long as the diaphragm is located in front of the front element. A lot of lenses have smaller aperture diameters than what they should have for a given focal length and aperture specified. This is because by placing the diaphragm on a different place the specific diameter of the aperture can be different but the amount of light would be the same. Here is a good explanation.

    I guess that constant aperture zoom lenses work by displacing elements or groups as you change the focal length. This allows the same amount of light to pass through the diaphragm by stablishing a different path for rays of light.

    But well, that's a guess

    Best regards,
    Sebas.
    Last edited by sebasj; 24th May 2008 at 05:33 AM.

  3. #3
    Raycer's Avatar
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    Calgary, Canada
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    Re: constant f/stop lens

    Thanks Sebas,
    I think you are right. I followed the link you provided and did more search on the internet. Looks like there's at least one group up front that gathers light before the light passes through the diaphragm.
    Gosh, how did people learn before internet.

    Cheers
    Ray

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