I posted this in another thread as a reply, but I think it has a
wider appeal, so I am reposting it as a thread in its own right,
and making a few clarifications.
[snip]
AF systems work to a specific precision. In the canon EOS system,
there are two precisions.
Normal Precision. This is the precision that the vast, overwhelming
majority of canon EOS cameras work to. To know what cameras work to
this precision, read the list of cameras which come under the high
precision category. If it's not on there, it's normal precision.
The important ones for this forum are to note that the D30, D60 and
10D (I'm making an educated guess...you'll see why) are all normal
precision cameras.
High Precision. The only cameras capable of high-precision AF are
the EOS 1, EOS 1N (and RS), EOS 3, EOS 1V, EOS 1D and the EOS 1DS,
and HP AF is not available with all lenses, depending on maximum
aperture. With variable-aperture zoom lenses, only the slowest
aperture value is noted. With the EOS 1, HP AF is enabled when a
lens of F2.8 or faster is used. With the EOS 1N, HP AF is enabled
on the central focus point only when a lens of F2.8 or faster is
used. On the EOS 3, 1V, 1D and 1DS, HP AF is enabled on the central
focus point on an F4 or faster lens, and then a further 6 points (3
above, 3 below) when an F2.8 or faster lens is used. Note that on
all of these cameras, HP AF is combined with cross-type focus
sensors.
Canon are notably vague on the difference between normal precision
and high precision. However (and I'll check on this, but this is to
the best of my knowledge), the best definition I know of is that
Normal precision will put the lens within 100% of the DOF from the
"perfect" point of focus. High precision will put the camera within
33%. The DOF used is that of the lens wide open (so, almost
paradoxically, the smaller the DOF, the closer you will be to ideal
focus). This is one reason that faster lenses give you better
focusing (the other, of course, being that they provide the focus
sensors with more light in a given situation).
Reference:
http://bobatkins.photo.net/info/faq30/eos3af.html
[
snip]
Clarifications: The EOS 10D is likely a normal precision camera
since it uses the 7 point AF system as used by the EOS 30, EOS 300
and EOS 300v, which are all normal precision cameras. The only
instance where what appeared to be the same focus system was
available in normal precision and high precision "flavours" was
that of the EOS 5 (Normal Precision) and EOS 1N (High precision as
previously described). However, the EOS 5's central AF point was
cross type to F5.6, whilst the 1N's was Cross-Type High precision
at F2.8 or faster only.
Variable Aperture Zoom lenses. Source of further confusion on the
cross-type/high-precision issue. Look at the central sensor of the
EOS 3/1V/1D/1DS AF system, and it is cross-type, high precision
(XTHP from now on) at F4 and faster. So what happens with a zoom
lens that crosses this boundary?
Let's take the popular 28-135 F3.5-5.6 IS USM as an example.
Obviously at the long end this lens can not provide XTHP focus.
However, at F3.5, the short end is fast enough for XTHP to work.
However, rather than have XTHP cut in and out, Canon decided to
disable XTHP with such a lens. Thus the 28-135 IS does NOT enable
XTHP, and neither does the 24-85 F3.5-4.5 for example. The 70-200
F4L DOES enable XTHP.
Note that with the older XTHP AF systems of the EOS 1 and EOS 1N,
an aperture of F2.8 was required, and so the 28-80 F2.8-4L USM did
NOT enable XTHP with these cameras, and was a major reason for the
demise of this lens in favour of the 28-70 F2.8L which does enable
XTHP.
--
Full Frame user!
EOS 3 + Nikon Coolscan III
It's good though to highlight this issue, as Photoshop's zoom function may well be making expectations fail for some.