The length of the focal length?

Discussion in 'Digital SLR' started by Desdemona, Oct 5, 2007.

  1. Desdemona

    Desdemona Guest

    Just for fun, I measured a 200mm lens from the front objective, to the camera
    film plane, and it was 200mm... nice... but I tried a 300mm lens and it was only
    230mm??

    What gives?

    Thanks!
     
    Desdemona, Oct 5, 2007
    #1
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  2. Desdemona

    Bob G Guest


    Telephoto designed lenses focus an image at a distance shorter than
    their focal length. That's their main virtue.

    Long lenses (such as your 200mm) for medium and small format cameras
    (including digital cameras) are typically of telephoto design.

    I'm surprised at the results you got with the 200mm. I suspect it's a
    large format lens adapted to a medium format camera.

    Can you give us the types and brands you were measuring?
     
    Bob G, Oct 5, 2007
    #2
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  3. What gives?

    Two properties of a lens: effective focal length versus back focal length.

    Not the same thing.
     
    Richard J Kinch, Oct 5, 2007
    #3
  4. Desdemona

    Paul Furman Guest

    Pinhole 'lenses' will follow that rule... which makes it pretty much
    impossible to do a wide angle pinhole shot on an SLR with a swinging
    mirror that can't belocked away while shooting (I tried with tin foil).
     
    Paul Furman, Oct 5, 2007
    #4
  5. Desdemona

    Mike Coon Guest

    Quite. My undergrad optics textbook (1st published 1957) defines the ratio
    of the two (though it uses "equivalent focal length") as the "telephoto
    magnification".

    Mike.
     
    Mike Coon, Oct 5, 2007
    #5
  6. Desdemona

    Bob G Guest


    That doesn't explain the OP's "discrepancies".

    Back focal length is measured from the rearmost element, focal length
    is measured from the nodal point and is given by the manufacturer,
    effective focal length is the focal length at a given focusing
    distance, as given by 1/F effective = 1/u + 1/v. You can see
    that F effective equals F when u (focusing distance) is at infinity.
     
    Bob G, Oct 5, 2007
    #6
  7. Desdemona

    Bob G Guest

    The focal length of a pinhole lens is equal to the distance from the
    pinhole to the film (or digital sensor, these days) plane.

    Paul is right, that WA pinhole photography is impossible with a fixed
    (as against a view) camera. I believe the distance between the film
    plane and the front mount on a typical SLR is of the order of 50mm, so
    that a pinhole lens is perforce 50mm or longer.
     
    Bob G, Oct 5, 2007
    #7
  8. Desdemona

    RichA Guest

    They all use negative lenses in the optical train to effect a longer
    focal length that the total length would imply. A tele-converter is
    in nearly every telephoto lens.
     
    RichA, Oct 5, 2007
    #8
  9. Your 300mm is definitely a telephoto lens, your 200mm is just a long
    focal length lens - the two are not the same, no matter how popular that
    misconception is.

    http://en.wikipedia.org/wiki/Telephoto

    You can also get inverted telephoto lenses where the distance from the
    rear element to the focal plane is longer than the focal length. Almost
    every wide angle lens for use on an SLR camera is an inverted telephoto
    lens.
     
    Kennedy McEwen, Oct 5, 2007
    #9
  10. It does explain, because it is the fundamental optic theory underlying why
    a lens system's physical length may be short than its focal length.
    No. In optics, the "effective focal length" is the distance from the
    principal point to the focal point. Object distance has no bearing. The
    words "effective" and "focal length" may be used in photographic
    discussions as you have cited, but that is not the "effective focal
    length" of optical theory.
     
    Richard J Kinch, Oct 5, 2007
    #10
  11. Desdemona

    Bob G Guest

    Object distance not only has bearing, it is impossible to ignore if
    you grant the Principal Law of Optics.

    When you speak of the focal point you must perforce be speaking of an
    image cast by the lens on that point. That image must be of an object
    somewhere in the physical world, and if so, it must be at some
    distance from the lens, which is the definition of object distance. No
    lens can form an image of nothing.

    Focal length is the distance from the nodal point of a lens focused at
    infinity to the focal plane. Effective focal length is the distance v
    from the nodal point to the focal plane when the lens is a distance u
    from the object focused upon. When u approaches inifnity, v (the
    effective FL) approches the FL.

    Interestingly, a lens cannot be focused on an object closer than its
    focal length: When u approaches F the effective FL approaches
    infinity.
     
    Bob G, Oct 5, 2007
    #11
  12. No. Consult any optics textbook.
     
    Richard J Kinch, Oct 5, 2007
    #12
  13. Desdemona

    Paul Furman Guest

    I tried an extension tube on a super wide angle lens and infinity was
    somewhere inside the lens :)
     
    Paul Furman, Oct 5, 2007
    #13
  14. As long as you use a symmetrical simple lens you can measure that way,
    but once you start using advanced lens designs you can measure it that way
    any more.
     
    Joseph Meehan, Oct 5, 2007
    #14
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