A&P Can someone explain attachable lenses to me?

Discussion in 'Lifestyle' started by Metaphysical, Mar 3, 2005.

  1. Metaphysical

    Metaphysical With a hundred ways to do a dozen things, why not

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    i know that the mm number is the range of the dof right??? what do the other numbers mean, and what else should i consider when buying a new glass?
     
  2. raptor_talon

    raptor_talon Who is the master? Sho'nuff!

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    wrong. the figures in mm are the focal lengths of the lense

    [​IMG]

    http://www.photoxels.com/tutorial_focal_length.html
    http://www.geometer.org/beginner/lenses.html

    from the above link:

    Imagine cutting a rectangle out of a card, holding the card in front of you, and looking at the world through the hole. Imagine that your photo will be what you see through the hole. If the card is held close to your eye, you'll see a great deal, and this is what happens with lenses with short focal lengths, which are also called "wide-angle" lenses.

    As you move the card away from your eye, this is equivalent to looking through lenses with longer and longer focal lengths, and when the card is quite far from your eye, you get a view similar to what you'll see through a telephoto lens.
     
  3. CRC

    CRC New Member

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    :eek3: Wow, that's an awesome analogy. Noob question, but what does the mm actually measure?
     
  4. bosox

    bosox *

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    " The focal length of a lens is defined as the distance in mm from the optical center of the lens to the focal point, which is located on the sensor or film if the image is "in focus"."
    [​IMG]
     
  5. Jcolman

    Jcolman OT Supporter

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    Some of the other numbers are the f/stops, distance and dof indicator. F/stops are usually expressed as 22 16 8 5.6 4 2.8. These are the settings that are used in conjunction with your shutter speed to control how much light hits the film or digital sensors. When you change shutter speeds you must change f/stops to mantain the correct exposure. Distance indicators is pretty self explanatory. It shows you the distance to your subject when you focus the lens. dof (depth of field) indicator shows you the approximate range of acceptable focus when your lens is focused on an object. It will vary depending on the f/stop selected.
     

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