--- Hugo_G�vert <[EMAIL PROTECTED]> wrote:
> On Fri, 2 Feb 2001, Julian Loke wrote:
>
> > Would there be any interest if Canon could devise
> an EOS 3 custom
> > function to retain cross-type AF down to f/5.6 or
> f/8?
>
> I'm pretty sure it would be a cross-type sensor if
> it was possible. I
> will not believe that those restrictions are for
> marketing reasons. I
> believe they are truly trying the get every inch of
> performance from the
> AF systems. Why EOS 50e (as well as EOS 7, 5 1N...?)
> has a cross-type
> sensor in the middle that works down to 5.6... I
> don't know.
There's no magic here. AF systems are subject to the
sames physical laws as manual focus systems. There's
a trade-off between accuracy and aperture in this
case.
Anyone who's old enough to remember manual focus SLRs
with a split-image "rangefinder" in the viewfinder is
probably familiar with what happens when you mount
slow lenses on the camers (or view with the lens
stopped down). For those without that experience,
I'll tell you: one half of the split-image blacks
out.
On most cameras, this happened at around f5.6. This
aperture isn't written in stone, but was chosen as a
reasonable compromise between focusing accuracy and
the need to work with slow lenses. I seem to recall
that my dad's Contaflex blacked out around f5.6, while
a friend's (much more expensive) Contarex did so
around f4.
The same thing happens to AF sensors, you just can't
see it in the viewfinder. The sensors in the EOS 50,
and other less expensive models, are set at a position
such that they'll remaain useful at smaller spertures,
at the cost of accuracy. That's a reasonable
decision, since the owner of an EOS 50 is more likely
to own lenses like the 70-300 than the 70-200 2.8 L.
For the "pro" oriented EOS bodies, however, like the
1, 1n, 1v and 3, Canon makes the assumption that the
owner is more likely to own fast, expensive lenses,
and is willing to pay for higher accuracy. But since
even pro's might use slower lenses for some purposes,
Canon compromises: the sensor is sensitive to f5.6 in
one orientation.
HTH,
Bob
=====
Bob Meyer
Life is uncertain. Eat dessert first.
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