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Thread: How do I calculate the mag ratio of lens with ET?

  1. #1

    Question How do I calculate the mag ratio of lens with ET?

    Recently found EF 17-40 F/4L can also go 1.02 with ET25 attached. My lens is only few centimeters from my monitor before fill an large icon full frame.

    How do I calculate the magnification ratio of any focal length, focus distance and ET 25mm?


  2. #2
    Member
    Join Date
    Dec 2004
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    Planet Earth
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    656

    Default Re: How do I calculate the mag ratio of lens with ET?

    Magnification ratio added = ET length / focal lengh.

    E.g. A 25mm ET on a 50mm lens will add on an extra ratio of 1:2 i.e. half. So, if the lens already has a magnification ratio of 1:5, then total magnification = 0.5 + 0.2 = 0.7.

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