Focal length...in plain language?

Discussion in 'Digital Photography' started by Jackson Bryan, Jul 24, 2007.

  1. Anyone able to explain focal length in plain language?

    JB
     
    Jackson Bryan, Jul 24, 2007
    #1
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  2. Jackson Bryan

    WalkingMan Guest

    On Jul 24, 7:31?am, "Jackson Bryan" <> wrote:
    > Anyone able to explain focal length in plain language?
    >
    > JB


    Distance from the lens to the film, in millimeters (Simple enough?).
    Due to the lens configuration, the physical distance is not always
    same as the stated length.
    Marshel
     
    WalkingMan, Jul 24, 2007
    #2
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  3. Jackson Bryan wrote:
    > Anyone able to explain focal length in plain language?


    From http://en.wikipedia.org/wiki/Focal_length:
    The focal length of an optical system is a measure of how strongly it
    converges (focuses) or diverges light. A system with a shorter focal length
    has greater optical power than one with a long focal length.

    jue
     
    Jürgen Exner, Jul 24, 2007
    #3
  4. Jackson Bryan

    Roy G Guest

    "Jackson Bryan" <> wrote in message
    news:f84nuc$cdf$...
    > Anyone able to explain focal length in plain language?
    >
    > JB
    >


    If the lens was a single piece of Convex Glass, then its Focal length would
    be the distance between the centre of the glass and a sharply focussed image
    of the Sun produced by it.

    The precise definition requires parallel rays of light falling on the lens,
    but for most practical purposes, the rays from the Sun can be considered as
    parallel.

    A Concave lens cannot produce a sharply focussed image of the sun behind it,
    so it ends up with a negative Focal length, but I can not remember how the
    figure is calculated.

    Every real life Camera lens has a number of pieces of glass within it, so
    the focal length is computed from a theoretical point, which may not even be
    within the physical lens barrel.

    Most Telephoto lenses are designed to be shorter than their effective Focal
    length, and most Wides are designed to be longer.

    Roy G
     
    Roy G, Jul 24, 2007
    #4
  5. On Jul 24, 6:31 am, "Jackson Bryan" <> wrote:
    > Anyone able to explain focal length in plain language?
    >
    > JB



    Maybe the "optical triangle will help. Draw a triange from the center
    of the lens to the edges/corners of the scene being photographed.
    There will be a similar triangle (same angles) from the center of the
    lens to the edges of the format (active area of the CCD). The object
    distance is the distance from the center of the lens to the objects in
    the scene. The image distance is the distance from the center of the
    lens to the image plane. In optics, to simplify the drawing, the
    "object" is usually considered to be an arrow perpendicular to the
    line from the lens to the center of the arrow.

    Now, if the object(s) is/are located at or near an infinite distance-
    that is, very far from the camera, the image distance will be the
    focal length. That is, the focal length of the lens is the distance
    from the lens to the image when the lens is imaging a very, very
    distant object.

    In anything other than macro photography, the image distance is just a
    little further from the lens than the focal length.
     
    Don Stauffer in Minnesota, Jul 24, 2007
    #5
  6. Jackson Bryan

    Allen Guest

    Jackson Bryan wrote:
    > Anyone able to explain focal length in plain language?
    >
    > JB
    >
    >

    For a simple lens (one element, or piece of glass) the lens is pointed
    at a far-away object (approaching infinity). Then a flat surface is put
    behind the lens and moved back and forth until the point is found at
    which the image of that object appears sharpest on that surface. The
    distance between the center of the lens and the focusing surface is
    measured; that distance (in millimeters, inches, feet, meters--any unit
    of length) is the focal length of the lens.

    With complex lenses (more than one piece of glass) it becomes more
    problematic. The _optical_ center of the lens must be determined. For
    fairly simple lenses (not extreme wide-angle, telephoto, or especially
    zoom) the optical center will usually be fairly close to the physical
    center. For those exceptions, which contain elements with _negative_
    focal lengths) the calculations become much more complex, and the
    average person is at the mercy of the lens designer or marketer as to
    where the optical center is located. The focal lengths of these lenses
    is actually "backed into" by the effect the lens has on the image size.
    For instance, going back to 35mm days, a lens with a focal length of 50
    mm was generally accepted as standard. A lens that produces an image
    twice as large as the 50mm is 100mm. Say a lens is marked as 500 mm:
    this means the image is twice as large, and the _optical_ center of the
    lens is actually half a meter (20 inches) in front of the physical
    center. This gets very, very complicated and quite often
    confusing--optics isn't a simple study.

    I hope this helps you rather than confuse you.

    Allen
     
    Allen, Jul 24, 2007
    #6
  7. Jackson Bryan

    JJ Guest

    The distance between a lens and the focused image it creates.
    JJ
     
    JJ, Jul 24, 2007
    #7
  8. JJ wrote:
    > The distance between a lens and the focused image it creates. (Of a
    > subject really really far away.) :)
    > JJ


    --
    Joseph Meehan

    Dia 's Muire duit
     
    Joseph Meehan, Jul 24, 2007
    #8
  9. Jackson Bryan

    AustinMN Guest

    On Jul 24, 6:48 am, WalkingMan <> wrote:
    > On Jul 24, 7:31?am, "Jackson Bryan" <> wrote:
    >
    > > Anyone able to explain focal length in plain language?

    >
    > > JB

    >
    > Distance from the lens to the film, in millimeters (Simple enough?).


    Too simple...that would mean the focal length changes with subject
    distance. It doesn't. A more accurate definition would be the
    distance between the optical center of the lens and the film plan
    *when focused on an object at infinity*.

    > Due to the lens configuration, the physical distance is not always
    > same as the stated length.


    Agreed.

    Austin
     
    AustinMN, Jul 24, 2007
    #9
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