F/number on a fixed aperture zoom lens?

Discussion in 'Beyond the Basics' started by prodigy2k7, Jul 26, 2008.

  1. prodigy2k7

    prodigy2k7 No longer a newbie, moving up!

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    EF 70-200mm F/4 USM
    The F/4 is constant. But the focal length changes. Does that mean the physical size of the aperture change?
    Ex: 70/4 or 200/4

    Isn't the physical size of the aperture the focal length divided by the aperture number?
     
  2. prodigy2k7

    prodigy2k7 No longer a newbie, moving up!

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    ha... Almost 20 views no replies!!!

    Either all the smarties are offline or they are lazy tonight :(
     
  3. Alex_B

    Alex_B No longer a newbie, moving up!

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    I never looked too much into lens internals, but it should, should it not?

    but then again, when the lens is wide open (f/4), aren't the aperture blades all retracted and out of the way? I guess then the aperture is only defined by the entrance pupil as given by the physical dimension of the lens front element?
     
  4. compur

    compur No longer a newbie, moving up!

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    As Bob Shell said in 2003:
     
  5. nymtber

    nymtber TPF Noob!

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    the f/number is a ration of focal length:apurture size (both in mm). So YES the ACTUAL size of the opening does change, but the same amount of light is let through.

    yea I learnt something in that Understanding Exposure book already :)

    Edit: Compur and I were typing at the same time apparently!
     
  6. Helen B

    Helen B TPF Noob!

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    There are a few threads about this. Here's one that prodigy started recently. The answer is in that thread - a constant aperture zoom lens may have a constant physical aperture (known as the aperture stop or sometimes as the iris) and rely solely on the change in magnification of the iris by the front elements; or it may have a variable physical aperture to complement the change in magnification. My Nikkor 17-35 f/2.8 is one of those designs that uses both properties to maintain its aperture.

    The important thing is that the f-number is the focal length divided by the diameter of the entrance pupil, not by the diameter of the physical aperture.

    Best,
    Helen
     
  7. epatsellis

    epatsellis TPF Noob!

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    Thanks Helen, you saved me from retyping my reply in the other post...

    erie
     
  8. prodigy2k7

    prodigy2k7 No longer a newbie, moving up!

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    Thanks everyone :)
     

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