On Nov 2, 2006, at 8:31 AM, K.Takeshita wrote: >> You know, I've been reading for years, from people I trust on this >> list >> and elsewhere, that the primary advantage of USM/HSM/SSM/whatever is >> that it's quiet rather than faster. No one should be surprised at >> this >> point. > > And perhaps the accuracy because of no overshoot.
Based on my experience with Canon USM lenses, the value of the USM in- lens servos is - they're matched to the particular lens requirements, so overall operation is more consistent across the range of lenses - they're quiet - they can be faster depending upon what you compare against. In the Canon line, there are only in-lens servos but the inexpensive line use simple servos, not the USM servos, and they are both slower and noisier. I haven't had any of the Nikon recent bodies that I could make any comparisons there. Overall focus speed and accuracy has more to do with the AF sensors in the body, how the system interprets the focusing criteria, how contrast the lens is when wide open, and what allowances for variance in focus accuracy are designed into the focusing algorithm. All AF systems are compromises in some sense. I expect the new generation Pentax DSLR bodies to improve nicely on the *ist D/DS generation in overall AF performance. I don't expect AF to be some kind of panacea ... I expect that I'll still switch to manual focus frequently to obtain the kind of critical focus accuracy that I want/need. The move to QuickShift as part of the focusing mount is, to me, a more important change than what kind of drive system the AF system uses. Full-Time-Manual-Focus is the single most rewarding part of the Canon USM lenses for me. Godfrey -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
