Lense speed?

Discussion in 'Photography' started by Jasuhn, Aug 19, 2003.

  1. Jasuhn

    Jasuhn Guest

    How do you know how fast a lense is?

    JASON
     
    Jasuhn, Aug 19, 2003
    #1
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  2. Jasuhn

    Jan Philips Guest

    That means the maximum aperture.
     
    Jan Philips, Aug 19, 2003
    #2
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  3. Jasuhn

    Rod O'Connor Guest

    Actually, a 1.4 is 4 times faster than a 2.8, don't forget f 2....
     
    Rod O'Connor, Aug 19, 2003
    #3
  4. Jasuhn

    J C Guest

    Check your math and I think you'll find that the f/1.4 is FOUR times
    as fast as the f/2.8. You cannot compare the f/stop number. You need
    to actually calculate the AREA of the aperture as follows:

    50 mm / 1.4 = 35.714 mm = diameter of the aperture
    and the area of this opening is calculated with the formula for
    calculating the area of a circle (pi x radius squared)
    So the maximum area of the aperture in a 50 mm 1.4 lens is
    3.14 x 17.857 x 17.857 = 1001 square mm

    and for the f/2.8 lens the calcuation is
    50 mm / 2.8 = 17.857 mm = diameter of the aperture
    so the area of this aperture is
    3.14 x 8.928 x 8.928 = 250 square mm

    1000 is four times 250 so the 1.4 f/stop lens lets in FOUR TIMES as
    much light as the 2.8 lens, not twice as much. A lens with a max
    aperture of f/2 would let in twice as much light as one with a max of
    f/2.8.

    By comparison, we know that at f/11 the shutter lets in twice as much
    light as it does at f/16. That is because a 50 mm lens at f/11 has an
    opening of 16.22 square mm, whereas at f/16 the opening is 7.66 square
    mm (and at f/22 the area is 4.05 square mm). Whereas your math above
    would lead one to believe that f/11 let in twice as much light as f/22
    and we know that is not true

    HOWEVER to be precise the above calculations do have rounding errors.
    AND it should be noted that as the aperature gets smaller it is less
    of a circle and more of a polygon so the above calculations using the
    area of a circle are slightly flawed.


    -- JC
     
    J C, Aug 19, 2003
    #4
  5. Jasuhn

    J C Guest

    NOTE that in this post I did not proceed to further elaborate that the
    diameter is only part of the comparison, ... because I assumed that
    people would have a basic grasp of geometry and therefore realize that
    the diameter had an impact on the area of the aperture. However, as I
    see in another post this is not the case and needed to be further
    explained.

    -- JC
     
    J C, Aug 19, 2003
    #5
  6. Jasuhn

    Dan Uneken Guest

    I would warn against using f 22.
    Stopping down the lens a few stops will eliminate or reduce many lens
    errors (aberrations) .
    But using very small apertures such as f 22 does not increase general
    sharpness in a picture, it increases depth of field (bringing nearer
    to be "in focus" those areas further away and closer to the camera
    that have not been focussed on), but not general sharpness. On the
    contrary, using the smallest aperture on a lens will almost certainly
    reduce sharpness due to diffraction of the light around the diaphragm
    edges. Every lens has its optimum sharpness somewhere in the middle
    aperture values, which may safely be estimated at around f 8 on 35 mm
    camera lenses.
     
    Dan Uneken, Aug 22, 2003
    #6
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