Calculating lens capability?

Discussion in 'Digital Photography' started by Digital Rebel XT EOS, Jun 1, 2006.

  1. A friend gave me some Kenko Extension Tubes - 36mm, 20mm, and 12mm -
    for doing macro photography using regular lenses.

    How do I determine which extensions or combinations of extensions to
    use? I've got a 28-135mm, 75-200mm, and 18-55mm lens...
    Digital Rebel XT EOS, Jun 1, 2006
    #1
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  2. Digital Rebel XT EOS

    Neil Ellwood Guest

    On Thu, 01 Jun 2006 07:52:08 -0700, Digital Rebel XT EOS wrote:

    > A friend gave me some Kenko Extension Tubes - 36mm, 20mm, and 12mm -
    > for doing macro photography using regular lenses.
    >
    > How do I determine which extensions or combinations of extensions to
    > use? I've got a 28-135mm, 75-200mm, and 18-55mm lens...

    Try them - use a spare morning, afternoon or evening.
    --
    Neil
    Delete l for email
    Neil Ellwood, Jun 1, 2006
    #2
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  3. Digital Rebel XT EOS

    tomm42 Guest

    Digital Rebel XT EOS wrote:
    > A friend gave me some Kenko Extension Tubes - 36mm, 20mm, and 12mm -
    > for doing macro photography using regular lenses.
    >
    > How do I determine which extensions or combinations of extensions to
    > use? I've got a 28-135mm, 75-200mm, and 18-55mm lens...


    Extension tubes don't work well with zooms, try it they may work, but
    you may have focus problems. Generally you need a 2 x focal length
    extension of the lens for 1:1, this is including the lens own
    extension. Also be sure you can meter with the tubes as the exposure
    will change.
    I expect you will have problems, but it is worth trying, and if you get
    the mag you need, thats great. Let us know how it works out.


    Tom
    tomm42, Jun 1, 2006
    #3
  4. Digital Rebel XT EOS

    Dankwart Guest

    The extension tube allows you to move closer to your object. It does not
    depend on the type of the lens but on the focal length you are working with.
    The smaller the focal length you are using, the more effective is a given
    extension tube.

    I did a quick calculation:
    When lens is set to infinity: D = F((F+E)/E)
    where D is the object distance, F is the focal length, and E the extension
    tube length, everything in the same dimension, like mm. When E=0, D is
    infinity, when E=F, your distance is twice the focal length F.

    When the lens is set to a closer distance you get even closer. (The equation
    becomes a bit more complex - I could give it to you.)

    In practice you may want to start out with the 12 mm if you want to work not
    too close. Then you use the longer extension tubes as you get closer. If the
    extension equals the focal length you get 1:1 life size, even when the lens
    is set to infinity.


    "Digital Rebel XT EOS" <> wrote in message
    news:...
    >A friend gave me some Kenko Extension Tubes - 36mm, 20mm, and 12mm -
    > for doing macro photography using regular lenses.
    >
    > How do I determine which extensions or combinations of extensions to
    > use? I've got a 28-135mm, 75-200mm, and 18-55mm lens...
    >
    Dankwart, Jun 16, 2006
    #4
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