----- Original Message ----- From: "Adam Maas" <[EMAIL PROTECTED]>
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.
I'm not denying that USM spin faster through the helicoid. I'm denying that "heavy optics in fast telephotos need improvement in the AF system". This is nonsense. The Pentax AF system may need improvement but not for that reason.
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.
Not true. If you look at the rear of any of your Pentax lenses you'll see that they have the same diametre... The heavy front tube of eg the 31 Limited needs to be moved when focussing, all the glass in the lens + the focus ring, all of it made of metal. In short you move virtually the whole bloody lens. Compare that to the few elements in an IF lens all of smaller diamtre than the said Limited lens.
The "heavy optics" aren't heavy to focus but to lift.
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).
Again I'm not denying that USM is faster. I'm denying that Pentax need updating the AF system due to the demand on the AF system provided by heavy optics. The heavy optics to focus are not fast telezooms that all have inner focusing mechanisms, but the ones without this fature all more "normal" lenses.
