How do different lens of varying focal length let same amount oflight at a given stop?

Discussion in 'Digital SLR' started by Anoop Saxena, Apr 18, 2009.

  1. Anoop Saxena

    Anoop Saxena Guest

    Hi,
    Consider two lens, 50mm and 100mm. At f2, the aperture diameter
    is 25mm and 50mm respectively. So how can they let the same amount of
    light? Am I missing anything here.

    Thanks in advance.
    Regards,
    Anoop.
     
    Anoop Saxena, Apr 18, 2009
    #1
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  2. Anoop,

    Same amount of light per unit area on the focal plane. Same "brightness",
    if you like.

    Given a simple scaling up of an optical design, the focal plane coverage
    of the larger lens will be four times the area (i.e. twice the linear
    dimension).

    David
     
    David J Taylor, Apr 18, 2009
    #2
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  3. The angle of acceptance of light of the lens, otherwise known as the
    width of the field of view, and linearly proportional to focal length.
     
    Chris Malcolm, Apr 18, 2009
    #3
  4. Anoop Saxena

    OG Guest

    At the focal plane the (4x as much) light is spread out over a (4x) larger
    area for the 100mm lens.
     
    OG, Apr 18, 2009
    #4
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