--- Tim Munro <[EMAIL PROTECTED]> wrote:
> ----- Original Message -----
> > With a CP on the front, you still have a f/2.8
> lens. The size of the
> aperture
> > has not changed. There will be less light getting
> to the front of the
> lens, and
> > therefore to the AF sensors. But it would be more
> accurate to compare it
> to
> > shooting on a cloudy day (when the sun is 2 stops
> weaker), instead of full
> > sunlight. If your AF slows down on a cloudy day,
> I would expect it to
> with a
> > CP.....................
> > Geoff Doane
>
> Sounds like a reasonable assessment there Geoff. I
> still can't quite fully
> understand the impact of the filter on the sensors.
> Your example is quite
> correct though. If the light level drops around 2
> stops it is hard to notice
> the difference in AF performance, whereas when I fit
> the 2x TC the
> difference is immediately obvious. As I shoot around
> 50 - 100 rolls a month
> at the track in demanding conditions I have become
> very sensitive to this
> aspect of my equipment.
> I will be interested to hear Mat's response as he
> sounds like he has had
> first hand experience with this.
The problem I have had is that the autofocus in AI
servo mode can't keep up with an oncoming car with a
polarizer on the lens. The lens and camera combination
(300 f4L on an EOS1) seems to focus fine on stationary
subjects with the circular polarizer on. Geoff's
conclusion makes sense, but doesn't match up with my
results.
I took a series of photos in Turn 2 (the Andretti
hairpin) at Laguna Seca in '99 at the American Le Mans
Series race there, some with the polarizer on, and
some with it off. I was using shutter priority, so the
camera would have corrected for the light loss by
opening the aperture. (A typical exposure at that
corner before noon is 500 at f8, you can't shoot there
in the afternoon as it gets backlit.) ISTR losing
about a stop by rotating the polarizer to the point
where the pavement looked black, so that would have
put the camera at its "lower limit" of f5.6 for AF.
I kept the focus sensor "bullseyed" on a high-contrast
item in the center of the frame, like a grille.
Everything looked fine thorugh the finder, but when I
got the pictures back, most of the polarizer pictures
had focus blur. The shots without the polarizer were
fine. (I do admit, though, that the colors were a lot
more saturated in the polarizer shots.)
I've used the same 77mm Tiffen circular polarizer on
my Sigma 135-400, and autofocus gets really marginal
with the polarizer on, it hunts a lot and can't seem
to acquire and track a target in AI servo mode. My
100-300 f4.5-5.6 USM has similar problems with a
polarizer installed. Focus is noticeably more
ponderous with the polarizer attached, and it becomes
useless for action shots in AI servo, you have to
anticipate where things are going to be an focus
manually.
YMMV if you have a newer camera/faster lenses, but
that's what I've noticed with the equipment I've
got... (EOS1/Elan II and the listed lenses) IMO a
polarizer seems to really degrade AF performance,
especially in AI servo mode. If there were really a
big improvement in this area with the EOS3 or 1v, that
might be a reason to upgrade... but, I do know that
the CMOS AF sensors don't work as well in lower light.
If someone can think of a way to make AI servo work
with my equipment and a circ pol, I'd like to know
their technique! :-)
MadMat
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