# 580ex buggy algorithm

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

1. ### John PaneGuest

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

2. ### Jim TownsendGuest

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

3. ### JBGuest

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