f2.8 vs f3.7- Is it one stop or 2 stop difference

Discussion in 'Digital Photography' started by zxcvar, May 17, 2004.

  1. zxcvar

    zxcvar Guest

    Greetings! What will the aperture stop difference between a lens with
    f2.8 at telephoto end compared with another with f3.7 lens at
    telephoto end.
     
    zxcvar, May 17, 2004
    #1
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  2. The next full stop after f2.8 is f4, so it's slightly less than a stop.
    The sequence goes in 1.414 steps, which is the square root of 2. This indicates
    the linear diameter of the aperture, and as the light passing factor is based on
    area of aperture, we get our 2x decrease per step. (Actual effective diameter
    is focal length / f number, so a 50mm f4 lens has an aperture 12.5mm diameter.)
     
    Malcolm Stewart, May 17, 2004
    #2
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  3. zxcvar

    Don Guest

    It's 0.8 stops.

    Don
     
    Don, May 18, 2004
    #3
  4. The others already answered your Q, but I'd add that determining
    actual brightness is not so simple. An f2.8 rating (or f-whatever) is
    based on pure geometry, things like glass transmissivity and advanced
    digital design may vary the brightness fairly significantly between
    two lenses at the same rated f-stop.

    Using a Sigma DG designated lens (not to be confused with DC), for
    example, is much like adding microlenses to a digital sensor because
    the rays exit more parallel which increases photon capture at the
    periphery of a digital sensor where the incidence angle is high. This
    brightens the image by as much as third to a half stop over older lens
    designs like all Canon L's (but it will have little to no effect when
    used over film).

    The bottom two images on this page show the brightness difference (0.3
    stops in this case) of a DG vs non-DG lens at the same rated aperture
    and focal length...

    http://www.pbase.com/imageprocessing/lenscompare
     
    Georgette Preddy, May 18, 2004
    #4
  5. The others already answered your Q, but I'd add that determining
    actual brightness is not so simple. An f2.8 rating (or f-whatever) is
    based on pure geometry, things like glass transmissivity and advanced
    digital design may vary the brightness fairly significantly between
    two lenses at the same rated f-stop.

    Using a Sigma DG designated lens (not to be confused with DC), for
    example, is much like adding microlenses to a digital sensor because
    the rays exit more parallel which increases photon capture at the
    periphery of a digital sensor where the incidence angle is high. This
    brightens the image by as much as third to a half stop over older lens
    designs like all Canon L's (but it will have little to no effect when
    used over film).

    The bottom two images on this page show the brightness difference (0.3
    stops in this case) of a DG vs non-DG lens at the same rated aperture
    and focal length...

    http://www.pbase.com/imageprocessing/lenscompare
     
    Georgette Preddy, May 18, 2004
    #5
  6. The others already answered your Q, but I'd add that determining
    actual brightness is not so simple. An f2.8 rating (or f-whatever) is
    based on pure geometry, things like glass transmissivity and advanced
    digital design may vary the brightness fairly significantly between
    two lenses at the same rated f-stop.

    Using a Sigma DG designated lens (not to be confused with DC), for
    example, is much like adding microlenses to a digital sensor because
    the rays exit more parallel which increases photon capture at the
    periphery of a digital sensor where the incidence angle is high. This
    brightens the image by as much as third to a half stop over older lens
    designs like all Canon L's (but it will have little to no effect when
    used over film).

    The bottom two images on this page show the brightness difference (0.3
    stops in this case) of a DG vs non-DG lens at the same rated aperture
    and focal length...

    http://www.pbase.com/imageprocessing/lenscompare
     
    Georgette Preddy, May 18, 2004
    #6
  7. zxcvar

    Lionel Guest

    Kibo informs me that (Georgette Preddy)
    stated that:
    Please ignore Preddy's gibbering - none of it relates to reality.

    PS: Where are those photos you lied about having sold, Preddy?
     
    Lionel, May 18, 2004
    #7
  8. Posting your story three times won't make it more believable...
     
    Bart van der Wolf, May 18, 2004
    #8
  9. zxcvar

    Mats Weber Guest

    The general formula for knowing the number of stops between two aperture
    values a1 and a2 is

    s = log(a2 / a1) / log(sqrt(2))
     
    Mats Weber, May 19, 2004
    #9
  10. Canon L glass is ok film glass (though absurdly priced and fragile),
    but it truly stinks for digital--very low tech--no Sigma "DG"
    counterpart.
     
    George Preddy, May 20, 2004
    #10
  11. zxcvar

    Lionel Guest

    Kibo informs me that (George Preddy) stated
    that:
    ....better than anything Sigma has ever produced.

    When are you going show us these photos you say you've sold?
     
    Lionel, May 20, 2004
    #11
  12. Or perhaps more simply

    s = 2 * log(a2/a1) / log(2)

    Dave
     
    Dave Martindale, May 20, 2004
    #12
  13. Less than one stop difference; the factor between full stop numbers is
    the square root of 2, which is 1.4 to one decimal place.
     
    David Dyer-Bennet, May 20, 2004
    #13
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