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  1. #11

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    Oh! I get it now, I wondered why my camera lens said 80mm. =)

  2. #12
    Leigh B's Avatar
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    Focal length is the distance along the lens axis from the rear node of the lens to the film when focused at infinity.

    The rear lens node is at the intersection of the lens axis and the second principle plane, which may be anywhere inside or outside of the lens. It's a design feature that's usually specified on the lens spec sheet.

    The rear lens node is the point within the optics from which all exiting paraxial rays appear to emanate.

    In contrast, the front lens node, at the intersection of the lens axis and the first principle plane, is the point on which all incoming paraxial rays appear to converge.

    The front and rear nodes are two distinctly different points. Both can be anywhere on the optical axis, and the rear node can be behind, on, or in front of the front node.

    In the Nikkor-T ED 270mm/6.3 for example, the first principle plane is 35.1mm in front of the front lens element, while the second principle plane is 9.6mm behind that, or 25.5mm in front of the front element.

    - Leigh
    Last edited by Leigh B; 08-26-2011 at 10:16 PM. Click to view previous post history.

  3. #13
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    Practically speaking, the longer the focal length, the narrower the angle of view.

    So a lens with a short focal length will have a wider view of things than a lens with a longer focal length.

    The actual, practical angle of view also depends on how big the film or sensor is. That is why a 50mm lens designed for 35mm film is considered a "standard" lens whereas a 50mm lens for an APS-C digital sensor (which is smaller than 35mm film) is a moderate telephoto (i.e. has a narrower than standard angle of view).

    In each case though, when focusing at infinity, the nodal point of the lens will be 50mm from the film/sensor.

    And a big 8x10 or larger view camera will require a much longer focal length lens to achieve the same/similar results.

    Binoculars and telescopes also have focal lengths. So do prescription glasses.

    EDIT: as pointed out below, questions of angle of view will only be directly related to focal length if your camera doesn't offer movements (such as are available in many large format view cameras)
    Last edited by MattKing; 08-26-2011 at 11:30 PM. Click to view previous post history.
    Matt

    “Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”

    Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2

  4. #14
    Leigh B's Avatar
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    Quote Originally Posted by MattKing View Post
    Practically speaking, the longer the focal length, the narrower the angle of view.
    I don't see where the OP asked about angle of view.

    Angle of view is not related to focal length except by general grouping. It's a function of lens design.

    - Leigh

  5. #15
    MattKing's Avatar
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    Quote Originally Posted by Leigh B View Post
    I don't see where the OP asked about angle of view.

    Angle of view is not related to focal length except by general grouping. It's a function of lens design.

    - Leigh
    Leigh:

    It seemed to me to be one of those "illustrations" that the OP asked for, that might help the OP understand what effect focal length actually has.

    Thus the word "Practically".

    And for most people, who don't have the advantage of camera movements, format plus focal length determines the angle of view.

    I'll agree however, that I should have confirmed that I was referring to cameras without movements - and will edit my post accordingly.
    Matt

    “Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”

    Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2

  6. #16
    Leigh B's Avatar
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    Quote Originally Posted by MattKing View Post
    And for most people, who don't have the advantage of camera movements, format plus focal length determines the angle of view.
    OK. I'll go along with the 'illustrations' thing.

    The focal length and format define the angle of view, and thus are the specifications for the lens design.

    The comment to which I responded earlier did not mention format.

    As an example, in 150mm lenses you can find AoV from 8.2° (Olympus Zuiko 150/2) to 106° (Nikkor-SW 150/8).

    And yes, for any given format, a longer focal length will give a narrower AoV. That's simple geometry. That's why we have wide-angles and normals and telephotos.

    - Leigh

  7. #17

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    The title says it all. If someone could explain to me what focal length is, maybe with illustrations or something. You know, dumb it down a bit.

    Quote Originally Posted by Leigh B View Post
    Focal length is the distance along the lens axis from the rear node of the lens to the film when focused at infinity.
    The rear lens node is at the intersection of the lens axis and the second principle plane, which may be anywhere inside or outside of the lens. It's a design feature that's usually specified on the lens spec sheet.
    The rear lens node is the point within the optics from which all exiting paraxial rays appear to emanate.
    In contrast, the front lens node, at the intersection of the lens axis and the first principle plane, is the point on which all incoming paraxial rays appear to converge.
    The front and rear nodes are two distinctly different points. Both can be anywhere on the optical axis, and the rear node can be behind, on, or in front of the front node.
    In the Nikkor-T ED 270mm/6.3 for example, the first principle plane is 35.1mm in front of the front lens element, while the second principle plane is 9.6mm behind that, or 25.5mm in front of the front element.

    - Leigh
    That covered it! Don't know about how basic it was.
    Heavily sedated for your protection.

  8. #18

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    The explanation with a simple magnifying glass is probably the best to understand the principle. With modern camera lenses however, it can be more complicated than that, as Leigh B tried to explain.
    Look up "retrofocus" and "telephoto" at Wikipedia. These are the most useful (understandable for someone with little knowledge of optics) articles I could find on the subject. It's about how it's possible that you can have a f=15mm lens on your SLR which is a lot farther away than 15mm from the film (which would be logical if focal lenght equals lens-to-film distance). Or how it's possible to build a f=500mm lens that's actually shorter than it's focal length.

    I like playing around with various lenses and some ground glass and found out that building complex lenses (with more than 1 element) isn't that hard. I once built a 30mm/1,3 lens for 35mm film cameras with some old glass... getting it to be reasonably sharp and free of optical aberrations (look those up too, if you're interested in lens construction) is the hard part that makes development of new lenses so difficult.

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