Newbee Question Regarding Lens Magnification

Discussion in 'Digital SLR' started by Sidney Friedman, Jan 11, 2006.

  1. I have two AF Nikon zoom lenses for my Nikon D70s, 18mm-70mm and 70mm-300mm.
    What exactly do these numbers represent?

    For example, does 300mm mean a magnification of 300 times? Does 70mm mean
    only 70 x magnification? Or....

    Thanks for clarification of the above.
    Sidney Friedman, Jan 11, 2006
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  2. Sidney Friedman

    Jack Dale Guest

    Or ... is the right answer.

    Check the following for a good explanation with pictures.

    Jack Dale, Jan 11, 2006
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  3. Sidney Friedman

    Jeremy Nixon Guest

    It's the lens focal length -- technically the distance from the optical
    center of the lens to the image plane (sensor). It is not the physical
    distance, because this is modified by the lens elements; conceptually,
    it is what the physical distance *would* be if there were no lenses at
    all, that is, a pinhole "lens" at that distance would project an image
    of that size.

    The relation to magnification is direct; 70mm is 2x magnification over
    35mm, for example.
    Jeremy Nixon, Jan 11, 2006
  4. Sidney Friedman

    C J Southern Guest

    You're sort-of on the right track - the bigger the number, the closer things
    will appear. Think of 50mm as being normal (that is if you take a photo with
    a 35mm camera using a 50mm lens then what you get will be pretty much what
    the eye sees without the camera - so a 300mm lens would be about a 6x zoom.

    Things get a bit more complicated when you use a camera with a smaller
    sensor (like I assume that the D70 has) - this has the effect of making the
    lens appear longer (or the image closer) - usually by a factor of around
    1.5 or 1.6.

    Take a look at ...

    For more info.
    C J Southern, Jan 11, 2006
  5. Thanks to Jack, Jeremy and CJ for claifying this question for me.
    Sidney Friedman, Jan 11, 2006
  6. Sidney Friedman

    tomm42 Guest

    Now to confuse the issue of a "normal lens". A normal lens is in
    relationship to the film size. It is the diagonal of the film, so a
    "normal" lens for a 35mm is 43mm or so. A 2 1/4 camera 80mm or so. A
    "normal" lens for an APS sensor would be approx. 28mm. The eye is
    really a different optical system than a camera, but the closest
    approximation in focal length is 80mm. I haven't found a good reason
    why 50mm became the defacto standard on 35mm but it did, a most boring
    focal length, made a little more interesting with APS sensors.

    tomm42, Jan 11, 2006
  7. Sidney Friedman

    Tony Polson Guest

    The reason is that lenses in the 50mm - 58mm range are the easiest and
    therefore cheapest to make.

    This is also the easiest focal length range for making optics of very
    high quality, which is why in most lens ranges, the 50mm (or 55mm or
    58mm) standard lenses were the best performers.
    Tony Polson, Jan 11, 2006
  8. Those are focal lengths; that's the most basic physical property of a
    lens (or lens system), and there are articles about it all over the
    net and in lots of books.

    They don't translate directlly into magnification or angle of view or
    anything else on their own. They can be translated into those things
    in combination with the size of the film/imaging sensor.
    David Dyer-Bennet, Jan 11, 2006
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