--- Johan Berglund <[EMAIL PROTECTED]> wrote:
>
> Gary Fisher wrote:
>
>
> > I just checked again. Although the AF assist
> > pattern from the 550EX covers the 7 AF points of
> > the EOS 30, it doesn't work on the top and bottom
> > AF point (ECF or manual selection). I thought it
> > did, but testing shows otherwise.
> >
> > I checked the 420EX and it does work with all 7
> > (to be expected I guess).
> >
> > I don't know why the 550EX doesn't do this, after
> > all it has an area AF assist pattern to suit the
> > EOS 3's 45 AF points which covers all 7 points
> > for the 30. Presumably some sort of backward
> > compatibility problem (as a former Nikon owner I
> > know all about this!).
>
> Hi,
>
> I think I've got the answer to this:
>
> The reason the 550EX AF-assist doesn't work for the
> upper/ lower AF-points of the EOS 30/33 & 300
> (Elan 7E/7 & Rebel 2000), is because they are
> horizontal AF-points (sensitive to vertical lines).
>
> The 550EX's AF-assist pattern, outside the center
> area, is made up of only horizontal lines, which
> means it's only effective for vertical sensors,
> which all of the EOS 3 & 1V's AF-points, except
> the center point are (when not used with a f/2.8
> lens or faster).
>
> So, though the light "covers" these AF-points of the
> 7-AF-points cameras, it's of no use to them, as
> these AF-points cannot focus on the pattern. That's
> why it's not used with them.
>
Hi John,
One of the things I did for Julian some time ago was
to compare the AF patterns for the 550EX and 420EX.
The 420EX has 3 vertical and 3 horizontal lines (close
together) in a cross pattern to suit the 7 af points
of the EOS 30 / 7e.
The 550EX has a "grid" display of horizontal and
vertical lines that cover the central AF area.
Admittedly, there are more (and longer) horizontal
lines than there are vertical, but the vertical lines
cover the top and bottom AF points of the EOS 30
adequately, particularly at 50mm focal length and up.
I can only conclude that it's a software limitation
(like the EOS 5 / A2e using the built in AF assist
rather than that of a 540EZ). That software
limitation may be based on the fact that the field of
view of a 24 or 28mm lens places the top and bottom AF
points slightly outside the range of the vertical AF
assist lines of the 550EX.
I noticed that the verticals on the 420EX are longer
(presumably) to accommodate a wider field of view down
t 24mm. When I initally tested this I was using a
50mm prime, so this wasn't obvious (deficient testing
methodology I'm afraid).
I seem to recall that the EOS 300 / Rebel 2000 can't
use the 550EX pattern for the top and bottom of it's 7
AF points either, quite possibly for the same reason.
Regards
Gary
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