K.Takeshita wrote: > On 9/01/06 9:29 PM, "Joseph Tainter", <[EMAIL PROTECTED]> wrote: > > >>O find that I can't get excited about having a motor in the lens. What's >>wrong with me? What am I missing? >> >>Is anyone else not worked up over this? >> >>It strikes me as one more thing that will need to be repaired >>someday...probably when parts are no longer available. > > > When we talk about USM, as Canon term it, it is an Ultrasonic Motor, and not > like a conventional motor. > USM is not Canon's creation and there are many companies making it for > various applications. Canon is probably the first to apply it for lens > drive. There is no motor per se, but you can consider the helicoids as the > rotor and the barrel as the stator, and the stator and the rotor are in > constant "contact". Inside the stator, there are a series of Piezo elements > which change its shape as the power is applied (like a wave) which push the > rotor. Unlike ordinary electric motor, which is typically high speed, low > torque machine and requires a gear reduction, USM is a low speed, high > torque device. Another advantage for the AF application is that it does not > require a brake as ordinary motors do, because the rotor and the stator are > in contact. This contributes to the fast start and move, and accurate > positioning due to no run-off of the rotor. > Reliability is excellent but it wears down due to the constant contact. But > the life (in terms of hour and I do not remember) is long enough for the > balance of life for most of us :-). > > I think Canon has another in-lens driver which uses step motor or something > but it's called differently. > > For further detail, I am sure Canon site should be providing it. > > Cheers, > > Ken >
To make life interesting, Canon has 3 different versions of USM and two other in-lens motor types. The original EF lenses were AFD (Arc-form drive) which used a wierd variation on a conventional DC motor (The motor was arc-shaped, hence the name). This design did well with small primes but had no performance advantages over screwdriver AF (since it's essentially an application of the exact same technology with the motor moved into the lens). On the low-end, AFD was replaceed by the micro-motor drive, which is faster with small, light lenses but not powerful enough for anything heavy. It's essentially a small high-speed motor. Then Canon introduced the USM Ken describes, with the focus ring being an electronic control (which was also the case in certain early AFD lenses, a couple even lacked MF capability entirely)meaning no focusing without power. These are the lenses that made Canon's reputation for fast AF. They would later be replaced by 2 different types of USM. The first is Ring-Type USM, which adds a clutch mechanism for mechanical full-time manual focus and works faster, it's essentially an improved version of the original USM, these lneses also provide distance info for flash exposure. The second is called micro-motor USM, and is merely a conventional micro-motor driven by ultrasonic AC. Faster than AFD or non-USM micro-motor drives (Mostly by driving the motor harder) but lacking the speed and Full-time manual focusing of Ring-type USM (Except for the 50mm f1.4 USM, which is cluched like the Pentax DA lenses, it's only micro-motor USM due to size constraints). All the low-end USM lenses except the 28-105 f3.5-4.5 USM are micro-motor. With the replacement of teh 85mm f1.2L this spring, IIRC all Canon USM lenses currently made are the latter two types. NIkon also uses the latter two types of USM, but calls it AF-S. Currently, all Nikon AF-S lenses are ring-type except for the 18-55DX and 55-200DX (the ultra-cheap digital kit lenses) which are micro-motor designs. Nikon also had two previous in-lens motor designs, the original F3AF lenses with a conventiional motor and the AF-I telephoto primes, which had a large matched AFD-type motor. All three use the same communication protocol, although only the F4 officially supports the old F3AF lenses (which aren't CPU lenses, so can't talk to modern bodies properly). -Adam -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
