How do constant f-stop zoom lenses work? | FerrariChat
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How do constant f-stop zoom lenses work?

Discussion in 'Creative Arts' started by blackwood, Mar 17, 2008.

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  1. blackwood

    blackwood Formula 3

    Dec 15, 2005
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    Redondo Beach, CA
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    #1 blackwood, Mar 17, 2008
    I'm trying to visualize the mechanism.

    Say I have my 24-105 f/4 set to f/4 at 24mm. When I zoom to 105, the lens opening must get bigger to remain at f/4. How does that work? Is the "screw" action that increases focal length also geared into the aperture blades? And what about push/pull zooms?
     
  2. MarkPDX

    MarkPDX F1 World Champ
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    Apr 21, 2003
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    #2 MarkPDX, Mar 17, 2008
    I would assume that the mechanisms work together... when I zoom on my 24-105 you can see the aperature changing. Can also see a little bit of the mechanism in the 70-200, not sure how to describe it though.
     
  3. blackwood

    blackwood Formula 3

    Dec 15, 2005
    1,822
    Redondo Beach, CA
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    #3 blackwood, Mar 18, 2008
    I found the answer elsewhere. The lens opening doesn't change (which, at first glance, implies that the f-stop does).

    However, what's important to the exposure is the "apparent aperture."

    When you zoom a lens, a the magnifying optics move relative to the aperture (diaphragm). This magnifies the aperture. Constant f-stop lenses are designed in such a way that the aperture and the focal length vary in a 1:1 ratio.

    Pretty slick.
     

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