zoom in mm

Discussion in 'Digital Photography' started by Not Me, Apr 30, 2004.

  1. Not Me

    Not Me Guest

    Can someone tell me how to convert a lens lenght in a zoom setting.
    Such as a 300mm lens would be what in zoom equivalent?
    Not Me, Apr 30, 2004
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  2. Not Me

    EdO Guest

    By it's self a 300 mm lens has no zoom equivalent, it has to be related
    to something else.

    If you assume that 50 mm is the "Normal" lens for a 35 mm camera then a
    300 mm lens for this camera would be a 6x magnification but is still not
    a zoom. To be a zoom the lens has to be infinitely variable from 50 mm
    to 300 mm.

    So zoom (x) is just a multiplier of minimum to maximum focal length for
    a given lens.

    EdO, Apr 30, 2004
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  3. Not Me

    Jim Townsend Guest

    A fixed 300mm lens would have a zoom of 1X.

    Lens zoom is used to describe a lens of variable focal length. The
    zoom figure (X) is determined by dividing the maximum focal length
    the lens can provide by the minimum focal length.

    Zoom is actually a verb describing the apparent to/from motion caused
    by changing the focal length of a lens.

    When you increase the focal length, objects in the viewfinder
    appear to move or 'zoom' towards you. When you decrease the focal
    length, objects appear to move or 'zoom' away from you.


    A 15-30mm lens would have a zoom of 2X (30/15 = 2)
    A 50-100mm lens would have a zoom of 2X (100/50 = 2)
    A 100-400mm lens would have a zoom of 4X (400/100 = 5)
    A fixed 300mm lens would have a zoom of 1X (300/300 = 1)
    All fixed (or prime) lenses have a zoom of 1X

    Note that the zoom rating of a lens has *nothing* to do with the
    magnification ability of a lens.
    Jim Townsend, Apr 30, 2004
  4. Not Me

    Tony Spadaro Guest

    If you mean "how does this focal length relate to a "normal" lens for the
    same camera, it will depend on what focal length gives a "normal" field of
    view. With 35mm cameras this is usually the 50mm focal length (43 actually
    but no one makes one) so a 300mm lens is a 6x magnification. With many
    digital SLRs the normal lens would be in the 30s.

    home of The Camera-ist's Manifesto
    The Improved Links Pages are at
    A sample chapter from my novel "Haight-Ashbury" is at
    Tony Spadaro, Apr 30, 2004
  5. Not Me

    vhl Guest

    If you put it this way, then all those people who think they have
    10x Zooms (eg. 38-380) will get shitty when you tell them that
    they only have a 0.75 - 7.5x zoom. (=

    I just hate it when anyone asks me what is the equivelant
    x times of my lenses.

    vhl, Apr 30, 2004

  6. Alright, enough people have addressed the usage of the term "zoom",
    and it seems fairly clear that the term you wanted to use was
    "magnification" or perhaps "power", as in binoculars and telescopes.

    The problem is, it all depends on how big your image area is, which
    is the film frame size, in film cameras, or the digital sensor size in

    Generally (subject to some variation), if you take the diagonal
    measurement of the image area, and add about 15%, you will get the focal
    length of what is "normal" for your camera, or basically, the same apparent
    magnification that your eye sees (which is safe to say, "none at all"). The
    image circle that comes out the back of the lens, in diameter, relates
    directly to focal length. Since the circle has to cover the rectangle of
    your image area (film or CCD or whatever), it has to be at least big enough
    to get across the longest measurement of that rectangle, which is the
    diagonal. Since lenses have light loss and greater distortion as you get
    closer to the edges of that circle, though, there is always an overlap
    designed into the camera, and this is typically about 15%. Every camera
    I've seen has hewn very close to this figure, but I have not done a
    comprehensive list.

    Thus what Tony Spadaro says about 43mm is accurate - add 15% and you
    get 49.45mm, close enough to 50mm to count.

    Divide the actual focal length by this 'guideline' number you have
    created, and you get the magnification or power. So for 35mm film cameras,
    a 300mm lens, divided by the 'normal' 50mm focal length, becomes a 6-power
    magnifier, or the same thing as a 6x scope.

    A digital SLR has a smaller image sensor, let's say 27.26mm diagonal
    measurement (this is Canon's Digital Rebel/300D, see
    http://www.dpreview.com/news/0308/03082005canoneos300d.asp). Add 15% to get
    31.35mm, so this is considered about 'normal', no magnification, and is
    your guideline. Call it 31mm - a third of a millimeter isn't going to make
    an appreciable difference.

    So divide your lens focal length by that, and you get your
    magnification or power. A 300mm lens becomes a 9.5x or 9.7x or so. As
    others have noted, don't confuse this with the manufacturer's idiotic zoom

    By the way, take that 31,35mm and multiply it by 1.6, and you get
    50.16mm, very close to the 'normal' for 35mm film. Surprise! Now you know
    where all that stupid stuff about multiplying focal length by 1.6 comes
    from. It only means that a 35mm film frame is 1.6 times larger than the
    digital sensor.

    Long winded, and it would have been easier simply asking what camera
    you have and going from there. But now everyone knows how to figure it for
    themselves, with any camera. Teach it to your children ;-)

    - Al.
    Al Denelsbeck, Apr 30, 2004
  7. All good stuff. To this, I would add that we're really talking about
    three somewhat different meanings of "X" or "times".

    For a zoom, the terminology "3X" just means that its longest focal
    length is 3 times its shortest. I could be a wide-to-normal zoom, or
    wide-to-moderate tele, or short-to-long tele, as long as it has a 3:1

    For a telescope or binoculars, "7X" is a measure of the angular
    magnification. If the subject is at infinity, and occupies "D" degrees
    of the naked-eye field of view, then when you look through a
    properly-focused telescope you see a virtual image at infinity that has
    been magnified 7 times to span 7*D degrees.

    With a camera, the actual magnification as seen by the viewer depends on
    the print size and print viewing distance. So you can talk about
    angular magnification, but you need to know these additional factors,
    which makes it not useful for characterizing lenses alone. Instead, the
    best we can do is talk about magnification relative to the "normal" lens
    for the format, which is just an arbitrary convention.

    But a camera lens forms a real image on a focal plane, while a telescope
    produces an aerial image at infinity, so they can't be compared
    directly. Now, you *can* often use a telescope with a camera by
    removing the eyepiece and mounting a camera in its place, and you *can*
    turn a camera lens into a telescope by mounting it to a special adapter
    with an eyepiece instead of a camera body. But the magnifications will
    be different when you do this, because the magnification depends on the
    eyepiece as well in telescope mode, and on the image sensor size while
    in camera mode.

    Dave Martindale, Apr 30, 2004
  8. Not Me


    Jul 22, 2011
    Likes Received:
    I know this is an old post but it still comes up and there is a error in the Something"X" to focal length range conversion. With 20X quoted on some cameras that would mean 18mm to 360mm if what was already said was correct, which would be some fantastic lens. But the 20X is talking about the area captured which is found using the square. So an 18mm to 80.5mm would be a 20X lens. So √18² x 20 = 80.5. So when my Sony Cine camera says optical 20X although good it is not as good as one may first think.
    ericmark, Jul 22, 2011
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