580ex buggy algorithm

Discussion in 'Digital Photography' started by John Pane, Mar 24, 2005.

  1. John Pane

    John Pane Guest

    Canon has embedded a buggy algorithm in the 580ex feature to account for
    the DSLR crop factor. The result is incomplete illumination of the frame
    at wide angles.

    The bug is in the relative order that the flash applies the crop
    factor and a threshold for the widest supported beam width.

    Defining some terms:
    f = the 35mm equivalent focal length of the lens
    d = the desired beam width computed from f

    The 580ex supports flash beam widths of 24, 28, 35, 50, 70, 80, and
    105mm (35mm equivalent).

    A correct algorithm for a 1.6 crop factor DSLR is:
    1) set d to f
    2) multiply d by 1.6
    3) if d is less than 24, set d to 24 (frame will not be fully
    illuminated)
    4) round d down to a supported beam width

    A buggy algorithm equivalent to what is embedded in the 580ex swaps
    steps 2 and 3:

    1) set d to f
    2) if d is less than 24, set d to 24 (frame will not be fully
    illuminated)
    3) multiply d by 1.6
    4) round d down to a supported beam width

    The correct algorithm would allow the 580ex to support any focal
    length from 15mm up without using the diffuser. The buggy algorithm
    only supports focal lengths from 24mm up.

    The following table shows the correct and buggy beam widths for a
    variety of focal lengths:

    f correct buggy
    ---- ------- -----
    15 24 35*
    18 28 35*
    24 35 35
    28 35 35
    35 50 50
    50 80 80
    70 105 105
    80 105 105
    105 105 105

    * Indicates frame not fully illuminated. In these cases the flash
    would work better (if not optimally) if no adjustment was made for
    the 1.6 crop factor.

    When using lenses wider than 24mm, I recommend setting C.Fn 11 to 1,
    disabling the crop factor adjustment.

    John Pane


    --
    John Pane, Mar 24, 2005
    #1
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  2. John Pane

    Jim Townsend Guest

    John Pane wrote:

    > Canon has embedded a buggy algorithm in the 580ex feature to account for
    > the DSLR crop factor. The result is incomplete illumination of the frame
    > at wide angles.


    Where are you getting this from ? Have you let Canon know
    so, if true, they can correct it ?

    If the info below is true, it only applies to the 20D and
    the new Rebel and the 1D MKII.. They are the only cameras
    that report the crop factor to the flash.


    > The bug is in the relative order that the flash applies the crop
    > factor and a threshold for the widest supported beam width.
    >
    > Defining some terms:
    > f = the 35mm equivalent focal length of the lens
    > d = the desired beam width computed from f
    >
    > The 580ex supports flash beam widths of 24, 28, 35, 50, 70, 80, and
    > 105mm (35mm equivalent).
    >
    > A correct algorithm for a 1.6 crop factor DSLR is:
    > 1) set d to f
    > 2) multiply d by 1.6
    > 3) if d is less than 24, set d to 24 (frame will not be fully
    > illuminated)
    > 4) round d down to a supported beam width
    >
    > A buggy algorithm equivalent to what is embedded in the 580ex swaps
    > steps 2 and 3:
    >
    > 1) set d to f
    > 2) if d is less than 24, set d to 24 (frame will not be fully
    > illuminated)
    > 3) multiply d by 1.6
    > 4) round d down to a supported beam width
    >
    > The correct algorithm would allow the 580ex to support any focal
    > length from 15mm up without using the diffuser. The buggy algorithm
    > only supports focal lengths from 24mm up.
    >
    > The following table shows the correct and buggy beam widths for a
    > variety of focal lengths:
    >
    > f correct buggy
    > ---- ------- -----
    > 15 24 35*
    > 18 28 35*
    > 24 35 35
    > 28 35 35
    > 35 50 50
    > 50 80 80
    > 70 105 105
    > 80 105 105
    > 105 105 105
    >
    > * Indicates frame not fully illuminated. In these cases the flash
    > would work better (if not optimally) if no adjustment was made for
    > the 1.6 crop factor.
    >
    > When using lenses wider than 24mm, I recommend setting C.Fn 11 to 1,
    > disabling the crop factor adjustment.
    >
    > John Pane
    >
    >
    > --
    Jim Townsend, Mar 24, 2005
    #2
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  3. John Pane

    JB Guest

    On Thu, 24 Mar 2005 06:01:45 +0000 (UTC), John Pane <>
    wrote:

    >Canon has embedded a buggy algorithm in the 580ex feature to account for
    >the DSLR crop factor. The result is incomplete illumination of the frame
    >at wide angles.
    >


    >When using lenses wider than 24mm, I recommend setting C.Fn 11 to 1,
    >disabling the crop factor adjustment.
    >


    John,
    I just tested my 20D and the 580ex. I came to the same conclusion. I
    got serious vignetting at 18mm. When I set cfn11 to 1, almost no
    vignetting. Have you contacted Canon?

    -JB
    --
    JB, Mar 25, 2005
    #3
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