Pål Jensen wrote:
----- Original Message ----- From: "Adam Maas" <[EMAIL PROTECTED]>
Pal, have you shot with a USM or AF-S lens? There's no comparison in
AF performance. USM absolutely blows screw-drive AF out of the water
unless you have either an ultra-light lens (like a 50mm) or one of
the very few cameras with a truly massive AF motor like the F5. I
simply could not believe the difference in AF performance between my
old *istD and Tamron 28-75 f2.8 and the EOS 3 with a 28-105 f3.5-4.5
USM lens. The EOS was so much faster I initially didn't believe my
own eyes (Although some of that likely has to do with having the same
AF unit as the EOS-1D). USM is needed in the Pentax line, badly.
I've tried the EOS-1v with the L 35-300 lens and this combo is as
"slow" as the MZ-S with the FA* 600/4.
I'm not denying that USM spins faster through the helicoid but when a
certain combination can follow virtually any subject you can point it
at with 4fps, what then is the point with faster AF unless you shoot
8fps?
Incidentally, Andy Rouse claims that the AF of the Pentax 645NII,
which uses SAFOX V, with the FA* 300/4 give just as many keepers as
the EOS-1v. USM is needed for marketing reasons. Spinnning fast
through the helicoid with lens caps on gives bragging rights in
brochures...
The 35-350 is a beast of a lens, as is the FA* 600/4, neither is going
to be as fast as a smaller lens, you should really pick a more
reasonable comparison than a pair of lenses that only a handful of
people will ever use. And the MZ-S does have a more powerful AF motor
than all but one or two other Pentax bodies, none of which are being
sold new. Compare a more common lens between the two (like I did) and
you will see noticable improvements with USM. Theres no reason a
relatively cheap 28-105 consumer zoom should outperform an IF 28-75 f2.8
for focusing, especially not as badly as the performance was (Some of
this is the AF unit, the 28-105 is typically in focus before the D had
even reached the ballpark, and sometime even before the D had started to
focus). As to how many keepers you get from a camera, well AF speed
makes no difference to that if you're waiting to get focus before taking
the picture. But the wait is what I'm complaining about, and it's very
apparent. This was an issue for me once I'd gotten a chance to see what
I was missing (The *istD wasn't noticably slower than the F90x, and
faster than the F801s or F65, which were my previous AF bodies).
I don't think the post you were responding to was talking about large
aperture zooms but rather fast AF zooms. Large aperture zooms have
bigger glass and are thus more difficult to drive for AF, on the
other hand, tehy do gather more light and usually have better
contrast, which helps the AF unit.
No. Large aperture zooms have large front elements that are not used
in focusing. Besides, most such lenses has inner focussing as well
making them easier to focus than, say, a consumer lens where whole
lens tubes + glass need to be moved in order to focus. . In fact, the
77 Limited is "harder" job on the AF system than a super telephoto IF
lens.
Large Aperture zooms have big glass all through the lens, as well as
that big honkin' front element. Yes, what's moved is lighter than the
largest element, but that doesn't mean that they are inherently easier
to drive than a cheapo lens, which is typically very loose and light.
Compare the AF performance of a Nikon 80-200 AF-S with the 80-200 f2.8
AF-D and you'll see the difference that ring-type USM provides. The
lenses are identical other than AF drive method, but the AF-S version is
consistently 20-30% faster to focus on an F90x, which is not lacking in
AF drive power (I've tried both lenses, noted the difference, but was
unable to buy either at the time, so I stayed with my big, slow 70-210 AF).
-Adam
