>
> > > Your make an interesting observation that the EOS 3 doesn't AF
> > > well with a lens slower than f/4!
I am sorry if I'm a little skeptical about this, but it certainly hasn't been my
experience using the equivalent of an f/5.6 lens--a 300/2.8L with 2x extender. Indeed,
the most serious problem with that combo is getting the subject properly in view. Once
targeted, AF works fine. I also have not had problems with the EOS 3 and EF
20-35/3.5~4.5 at 35 mm (f/4.5), or with the 200/2.8L and either the 1.4x extender or
25 mm extension tube (both producing an effective f/4 lens). I
suspect that the slower lenses are the culprit here, not the EOS 3. As someone
(Karen?) observed, the big zoom has slow AF components, and with the 28-135 IS,
perhaps the IS function, combined with its small maximum aperture(s), would slow down
the AF regardless of what body was used. I can't prove this, though. :-)
> I think the central sensor
> > > only detects horizontal lines when using your 75-300 lens or
> > > the 28-135 IS
The text regarding focus sensors in the EOS 3 "Technical Overview" is not at all
clear, so instead of trying to figure it out, I'll just quote it:
Each of the 7 CT (cross-type) AF sensors in the center contains a pair of
horizontal line sensors that operate with maximum apertures of f/2.8 or larger, and a
pair of vertical line sensors that operate with maximum apertures of f/5.6 or larger.
Also, the center CT sensor can be used even when using lenses with maximum apertures
down to f/4. And the center vertical line sensor operates with lenses with maximum
apertures down to f/8 for the first time ever.
So, it would appear that six of the CT sensors detect both horizontal and vertical
lines with lenses with maximum apertures of f/2.8 or greater, and only vertical lines
with lenses with maximum apertures of f/5.6 or greater; while the center CT sensor can
detect both horizontal and vertical lines with lens of only f/4 or greater and only
vertical lines with lenses with maximum apertures smaller than f/4 down to as small as
f/8. UNLESS by "vertical line sensors" they mean sensors
aligned vertically rather than sensors that detect vertical lines, in which case you
could only have detection of vertical lines at f/2.8 or greater (or f/4 or greater
when using the central CT sensor)--in other words, horizontal and vertical would in
that case be reversed in what I've written. (Based on this sample, I think Canon needs
to hire me--at a good salary--to edit their documentation).
Actually, the EOS 3 manual describes it more clearly:
1) all 45 sensors are sensitive to horizontal lines
2) the seven CT sensors are sensitive to vertical lines as well at maximum apertures
of f/2.8 or larger
3) with a maximum aperture of at least f/4 but smaller than f/2.8, only the center CT
sensor is sensitive to vertical lines; the other 44 sensors are sensitive to
horizontal lines only (with the exception of the 70-200/2.8L with 1.4x
extender--although the nature of the exception isn't specified)
4) with lenses that have a maximum aperture of f/6.7 to f/8 *with an extender
attached*, only the center sensor is active, and it is sensitive to horizontal lines
only.
5) Interestingly, it doesn't say anything about lenses whose "native" maximum aperture
is smaller than f/4 but at least as large as f/8: for example, it mentions the
400/5.6L with 1.4x extender, but not by itself. In other words, no mention is made
regarding at what maximum aperture the horizontal-line sensors (other than the center
sensor) become inactive. However, a quick, informal test using the 20-35/3.5~4.5 shows
that the the peripheral sensors are still sensitive to
horizontal lines at 35 mm - f/4.5, so perhaps they don't go inactive until f/5.6 or
even f/6.7.
Unfortunately, the 1V "Technical Overview" is as obscure on this point as is the one
for the EOS 3. Here's what it says:
. . . seven high-performance cross-type AF sensors that operate with a maximum
aperture of f/2.8. However, for vertical-line focusing, the EOS-1V employs a sensor
that operates with an aperture of f/5.6. Canon had good reason to choose this sensor
for vertical-line focusing: when presented with a uniform level of de-focus, an f/5.6
AF sensor suffers less focal divergence than an f/2.8 AF sensor. Thus, this system
offers better subject-movement prediction. The cross-type
sensor achieved both accuracy and improved subject-movement prediction, playing
an integral role in the EOS-1V's high-precision AF system.
Got that? I suspect that the technical writer who wrote this didn't fully understand
whatever explanation was offered up by the engineers--perhaps someone with a 1V could
check the manual for a less obscure explanation. . . .
fcc
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