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Thread: Mmmm...what is 'mm'?

  1. #1
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    Default Mmmm...what is 'mm'?

    Ok...one of those things I took for granted until someone asked me recently...


    ...how to explain the 'mm' used to determine the focal length of a lens? e.g. 50mm - thickness of the glass?

    Hmmmm...

  2. #2
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    Default Re: Mmmm...what is 'mm'?

    You have already answered your own question when you ask the question.

    How is mm used to determine the FOCAL LENGTH of a lens. e.g. 50mm.

    Thus, the FOCAL LENGTH of the lens is = 50mm.
    Andy Ang :lovegrin: - "A Photo speaks a thousand words. Have you spoken today?"
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  3. #3
    Senior Member creampuff's Avatar
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    Default Re: Mmmm...what is 'mm'?

    Distance between centre of the optical group to its focal point measured in millimeters.

  4. #4

    Default Re: Mmmm...what is 'mm'?

    millimetre

  5. #5

    Default Re: Mmmm...what is 'mm'?

    mei mei
    Last edited by morningstar; 12th June 2007 at 05:45 PM.

  6. #6

    Default Re: Mmmm...what is 'mm'?

    Quote Originally Posted by creampuff View Post
    Distance between centre of the optical group to its focal point measured in millimeters.
    When parallel rays of light enters the lens, it's the distance at which the rays converges to form a point. For a simple lens, this is the distance measured from the centre of the glass to the focal plane. For compound lenses, the effective focal length is more of a measure of the equivalent angle of view a simple lens of the same focal length would give when brought to focus over the same area.

  7. #7

    Default Re: Mmmm...what is 'mm'?

    Picture worth a thousand words.

    Top is short focal length, bottom is long focal length.

    http://www.gutenberg.org/files/16593.../image_072.jpg

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