Something I don't get about zoom lenses.

Discussion in 'Digital Photography' started by no_one_cares, Jun 3, 2011.

  1. no_one_cares

    no_one_cares Guest

    I've read that the magnification of a lens is calculated by
    dividing it's larger zoom number by it's smaller zoom number.

    If true, an 18 - 200mm lens has a magnification (not
    considering crop factor) of 11.1...

    But a 70 - 200mm lens would only have a maximum magnification
    of less than 3 times?

    Did I miss something?

    If that's true, why would anyone buy a 70 - 200 in preference
    to something that can better pull in distant objects?

    Obviously, I haven't yet taken a photography course or this
    would have been one of the first questions I'd have asked.

    Any help is appreciated.
    no_one_cares, Jun 3, 2011
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  2. no_one_cares

    charles Guest

    200/70 has nothing to do with magnification, it is the zoom range.
    Magnification would be determined by the 200mm or the 70mm, the larger
    the focal length number the greater the magnification. Some people
    like a greater zoom range, it means fewer changes on lenses, fewer
    lens purchases. Some people, like me, believe that lenses with very
    great zoom range are not as good optically.
    charles, Jun 3, 2011
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  3. no_one_cares

    TheRealSteve Guest

    You're confusing zoom range with magnification. Zoom range is easy.
    It's just the larger focal length / smaller focal length.

    Magnification is a little more complex. For a telescope or binoculars,
    it is the focal length / effective exit diameter, which is the
    eyepiece focal length. That's all well and good because you are
    looking right through the lenses at an object. With a camera, it's a
    little different because it's projecting onto a sensor. You can take
    the exact same lens and it will have different *effective*
    magnification for cameras that have different sensor sizes. This is
    true even though the actual magnification of the lens hasn't changed,
    and is the focal length / effective exit diameter.

    It gets a little more complicated for many zoom lenses with internal
    focus that have a small min focus distance. Take the 18-200 as an
    example. When you focus close, say less than 10', with the lens zoomed
    out to 200mm, it isn't really a 200mm focal length. For close focus,
    the lens compensates for the fact that it's length is restricted due
    to internal focus by reducing the focal length. That lens can't get
    much more than 140mm when the subject is close. At infinity focus, it
    does go out to 200mm. So don't believe your zoom ring markings unless
    the subject is far away.

    I have a quick and dirty calculus for lens magnification which relates
    to the "fact" that a 50mm lens on a 35mm camera is considered to be
    about 1x magnification. You just take your focal length and divide by
    50. Thus, a 200mm focal length would be a 4 power lens if you wanted
    to relate it to something like binoculars magnification.

    If you're shooting with an aps sensor that has a crop factor of 1.5,
    just take whatever you get above and multiply it by 1.5. So on a Nikon
    APS sensor, a 300mm lens would be a 1.5 * 300/50 = 9x lens and would
    have the same magnification as 9x binocs.

    There are more detailed analysis you can get into if you're really
    interested. Things like the lens reproduction ratio, which is
    dependent on subject distance and therefor ability to focus at
    distance X. Just google reproduction ratio and lens focusing formulas
    if you really want to get into it.

    TheRealSteve, Jun 3, 2011
  4. no_one_cares

    DanP Guest

    Meaning a picture taken at 200mm compared to one at 18mm is enlarged
    11.1 times.
    Think of it as zoom range.
    You get the same magnification/view at 200mm. And lose the wide view
    at 18mm.
    Marketing concentrates on megapixels and zoom range (x10, x30 etc).
    These matters least. Look at aperture before anything, sometimes
    called the f number, the lower the better.
    It means the lens will let a good amount of light in.

    I guess you own a good camera with interchangeable lenses. Take some
    shots in Av (aperture) mode at various settings.
    After getting my camera with the kit 18-55mm lens I have rushed and
    bought the 55-250mm. Then I got a 50mm f/1.8 prime (no zoom).
    I use the 50mm most and 55-250mm least. I would suggest you get the
    50mm prime first as it is the cheapest and a joy to use.

    DanP, Jun 3, 2011
  5. no_one_cares

    RichA Guest

    In 35mm terms, you can consider a 50mm lens to be essentially 1:1 and
    on an APS sensor to be 1.5:1 or 1.5x magnification. On a 35mm sensor
    for every 100mm, you are adding 2x. So a 200mm lens magnification
    would be 4x a 300mm lens would be 6x. You simply multiply that
    magnification by the sensor crop (1.5x, or whatever) and you have your
    effective magnification. A 500mm lens on an APS (1.5 crop) sensor
    would be: 500x2 = 10 x 1.5 (the crop) = 15x.
    This is all more or less arbitrary, the only important thing being how
    one focal length relates to another, 1000mm is 5 times the
    magnification of 200mm, etc.
    RichA, Jun 3, 2011
  6. no_one_cares

    Martin Brown Guest

    I suggest you stop reading that source then. The ratio of longest focal
    length to shortest focal length is the zoom *range*.

    The magnification of a lens depends on the format of the camera used.
    But for a 35mm slide camera is approximately 50mm = 1x, and about 0.7x
    that = 35mm for a typical digital SLR. For larger formats the lenses
    have longer focal length for a given magnification and for small APS
    point and shoots much shorter lengths and smaller sensors.
    Not magnification zoom range.

    Because there are extremely well optimised designs for zoom lenses that
    over a 3x range work almost as well as a fixed length prime lens.

    To get a 10x zoom range some design compromises on either vignetting,
    edge/corner sharpeness, barrel distortion and lateral colour have to be
    made. These days with software correction of certain aberrations on
    digital cameras this compromise is not so bad as it was on film where
    you were stuck with whatever image the lens put down.

    People still use fixed focal length lenses too (zoom range = 1). Such
    lenses can be optimised to be sharper than any zoom (although the
    difference between zoom optics and fixed length lenses is much less now
    than it was in the past). Zooms used to be pretty dire in the distant
    past going soft at long lengths and curvy barrel effects at short.
    If you are trekking then weight matters and one 10x zoom instead of a
    rucksack full of glass might be quite tempting.

    Martin Brown
    Martin Brown, Jun 4, 2011
  7. no_one_cares

    no_one_cares Guest

    Thank you.

    Now I'm gonna see if I can find the web page that tells people
    that the magnification is the lenses' large number divided by it's
    small number and thank the author for not knowing what he's talking

    I had to wonder, because it didn't make a lot of sense to me
    to have various lenses ending at 200mm and a cornucopia of smaller
    minimum numbers.
    no_one_cares, Jun 5, 2011
  8. no_one_cares

    no_one_cares Guest

    That's what I didn't get, because I did a search and some
    idiot with access to the internet and web authoring software (actually
    more than one) decided to tell everyone that a lenses' magnification
    ability is calculated by dividing the lenses' maximum focal length by
    it's minimum.

    With an 18-200mm lens, it would work out to a bit more than
    11x, if the camera has no "crop factor", and with a 70-200mm, it would
    be less than 3 power. It struck me as idiocy that companies would
    produce so many lenses with varying minimum focal distances, so I
    decided to ask here... and am glad I did.
    Oh yes. Once I found out about aperture settings, and checked
    out the prices of F3.5 lenses vs. f2.8 lenses on ebay and other sites,
    I became well aware of at least that difference.

    Thanks for the explanation.

    Now, to find the halfwit someone taught to use a computer...
    no_one_cares, Jun 6, 2011
  9. no_one_cares

    no_one_cares Guest

    If I start doing portraits a lot I'll get a prime lens with a
    large aperture, but for now I want to be able to bring objects farther
    away a bit closer and still have decent performance in low light.

    Perhaps NASA will release some optics and electronics combos
    that let us take shots in near total darkness with a 40x zoom in a
    package you can carry around in your pocket...
    no_one_cares, Jun 6, 2011
  10. []
    ... likely you can already do something near to that, given a tripod and
    long enough exposure.

    David J Taylor, Jun 6, 2011
  11. no_one_cares

    no_one_cares Guest

    Well, that's true to a degree.

    The D3000 is noisy above 400 ISO, but I've taken a long
    exposure at night in what I thought was very little light and it
    showed a lot of things I was very surprised at.

    I also managed to catch some lightning near the end of a 10
    second exposure a while back and it was a nice shot.
    no_one_cares, Jun 7, 2011
  12. no_one_cares

    no_one_cares Guest

    I mean where you end up getting the image magnified 40
    no_one_cares, Jun 7, 2011
  13. no_one_cares

    no_one_cares Guest

    Well I don't think I'll be buying one anytime soon if that's
    the case.

    So the "magnification" on the lens would not be 200mm divided
    by 18 then, if I understand what you're saying.

    Is there a formula then by which magnification can be

    And thanks for the info. This will help when I determine which
    of the zooms I can afford will best suit my needs.
    no_one_cares, Jun 8, 2011
  14. no_one_cares

    Whisky-dave Guest

    I think the term magnification is what is confusing things.
    We could confuse this further by mentioning telescopes, binoculars and
    microscopes :)

    Then we could compare magnification with field of view too.

    If the OP said what sort of things he wants to shoot then maybe some
    of what a specific lens would be best might more useful than these
    definitions and calculations.
    Whisky-dave, Jun 8, 2011
  15. no_one_cares

    Martin Brown Guest

    And just to make life arbitrarily more confusing some camera makers have
    become totally inconsistent and describe digital camera lenses (crop
    DSLRs and P&S cameras) with their lenses "notional" 35mm full frame
    equivalent focal length. This makes comparisons much harder even for
    people who know what they are looking at.

    A workable heuristic definition for effective magnification is given by
    focal length of lens divided by 1.5 x sensor diagonal length. It is
    crude but close enough to be useful as a rough guide.

    As SavageDuck says 50mm f.l. lens on a 35mm format is ~1x

    Anything much longer than 300mm on a 35mm camera and you will also need
    a decent tripod and/or image stabilisation.

    Martin Brown
    Martin Brown, Jun 8, 2011
  16. no_one_cares

    M-M Guest

    I'm sure you don't really mean that either. Consider that an image of
    a 6' man would be 240' high. What a camera![/QUOTE]

    40X magnification would be 2000mm. Here is ~45X magnification (1500mm x
    1.5 crop factor):
    M-M, Jun 8, 2011
  17. no_one_cares

    no_one_cares Guest

    That clears things up a bit. Thanks.
    no_one_cares, Jun 8, 2011
  18. no_one_cares

    M-M Guest

    Whatever it is you have calculated, it is not magnification.[/QUOTE]

    Ok then, what is it?
    M-M, Jun 8, 2011
  19. no_one_cares

    me Guest

    me, Jun 8, 2011
  20. no_one_cares

    M-M Guest

    I'm not quite sure. I don't understand what it is you think you are

    If 50mm is 1x magnification then 1500mm is 30x and then add in the 1.5x
    crop factor.

    The lens states it is 1500mm.
    M-M, Jun 9, 2011
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