how to determine lens magnification

Discussion in '35mm Cameras' started by drs, Jan 3, 2005.

  1. drs

    drs Guest

    How is the magnification calculated for lenses in terms of X-power?
    With 50mm focal length normal for 35mm (not considering crop factor on
    dslr), is 100mm 2X, 300 6X, etc.?
     
    drs, Jan 3, 2005
    #1
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  2. drs

    Peter Irwin Guest

    Magnification is normally used in photography to mean the ratio
    between object size and image size. This is not what you seem to mean.
    I think the question is about making the photograph look the same
    size to the eye as the original scene did from the camera position.

    The image in a photograph will look the same size as the scene did
    from the camera position if:

    viewing distance = focal length x englargement magnification

    (Technically it should be the distance between the film plane and
    the rear nodal point of the lens instead of focal length, but using
    focal length is a good enough approximation except for extreme
    close-up photography)

    So if you use a two inch lens and print a 5X enlargement (5x7
    inch print) the image in the photograph will look the same
    size as it did from the camera position if you hold the picture
    10 inches away.

    If you use a 300mm lens (about 12 inches) and also make
    a 5x7 inch print it will appear 6x life size if held 10 inches
    away. If you look at the print from five feet away, the image
    will look the same size as what you saw from the camera
    position. If you look at a 1x1 1/2 inch contact print from
    one foot away the image will also look the same size as the
    scene you saw from the camera position.

    If you enlarge the image from the negative taken with the
    2 inch lens to 11 x 14 inches (11 times enlargement) the
    image will look 2.2x the normal size when viewed from
    10 inches away, and will look normal sized when viewed
    from 22 inches away.

    I hope this helps.

    Peter.
     
    Peter Irwin, Jan 3, 2005
    #2
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  3. drs

    Tony Guest

    While lens magnification is not generally used in photography you can
    assume roughly that 50mm equals 1:1. in 35mm. But this really is about the
    apparent image size in a theoretical camera as many cameras these days have
    a smaller image in the viewfinder to save size/weight and to increase
    brightness.
    If you wish to be fussier with the math, then a 43mm lens would be 1:1
    in that same theoretical slr.
    Remember that this has only to do with apparent image size when viewing
    through the camera.
     
    Tony, Jan 3, 2005
    #3
  4. drs

    drs Guest

    I didn't mean enlarging after the fact. I'm trying to equate the power
    of a camera lens to that used to describe the optics in telescopes or
    binoculars. What lens, for instance, would give me a magnification of
    ten, similar to a 10X42 binocular? Teleconverters list the
    magnification power, typically 1.4X or 2X. So if I add a 1.4X to a
    200mm lens I obvious get a focal length equivalent of 280mm. But what
    magnification did the 200 lens have?
     
    drs, Jan 4, 2005
    #4
  5. Either Peter is not awake or he got up on the wrong side of the bed, or
    maybe he is some kind of language nut. Your question was just fine and
    clear. Your use of the term magnification is acceptable. The rest of the
    world uses it that way.

    I won't bother to repeat what Tony wrote as he did a good job.
     
    Joseph Meehan, Jan 4, 2005
    #5
  6. If accuracy is not paramount, then this works fine. A 500mm lens
    provides roughly 10X magnification.

    This runs into lots of other problems if you're trying to be
    accurate, though, among them "true" focal length (which often disagrees
    with what the lens is sold as), focal distance, and viewfinder
    magnification.


    - Al.
     
    Al Denelsbeck, Jan 4, 2005
    #6
  7. How is the magnification calculated for lenses in terms of X-power?
    100mm will make it seem like you're twice as close as 50mm, yes, so
    it's like a 2X monocular.

    (But "magnification" is usually used to mean how much larger, or
    smaller, the image will appear on the film plane than in real life,
    while "focal length" is used for what you seem to be calling
    magnification. For example, 1:1 magnification means that the actual
    image on the film or digital image sensor is the same size as the
    object you are photographing. This 1:1 magnification can be
    accomplished at any focal length, and is determined by the minimum
    focusing distance of a lens.)

    -Joel
     
    Dr. Joel M. Hoffman, Feb 27, 2005
    #7
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