Abbreviated as USM, and associated with digital camera terminology, ultrasonic motor (also called ultrasonic focus motor ) is a term that is mainly used to describe the motor that controls the movement of the lens or focus ring. An ultrasonic focus motor is much quieter (almost near-silent) and also moves the focus lens or ring faster when compared to a standard motor. A USM is able to hold the lens in place with precision and accuracy at the exact instant the lens arrives at the correct focus when using the camera's auto focus (AF) feature.
The technology has been applied to photographic lenses by a variety of companies under different names:
Canon - USM, UltraSonic Motor
Nikon - SWM, Silent Wave Motor
Minolta, Sony - SSM, SuperSonic Motor
Olympus SWD, Supersonic Wave Drive
Panasonic XSM, Extra Silent Motor
Pentax - SDM, Supersonic Drive Motor
Sigma HSM, Hyper Sonic Motor
VC, Vibration Compensation
Ultrasonic motors differ from piezoelectric actuators in several ways, though both typically use some form of piezoelectric material, most often lead zirconate titanate and occasionally lithium niobate or other single-crystal materials. The most obvious difference is the use of resonance to amplify the vibration of the stator in contact with the rotor in ultrasonic motors. Ultrasonic motors also offer arbitrarily large rotation or sliding distances, while piezoelectric actuators are limited by the static strain that may well be induced in the piezoelectric element.
There are two types of USMs, the ring-type USM and the micromotor USM. Ring-type USM allows for
full-time manual focus operations without switching out of AF mode. Micromotor USM is used to bring down the cost of the lens. It is possible to implement full-time manual focus even with micromotor USM; however, it requires additional mechanical components.
Dry friction is often used in contact, and the ultrasonic vibration induced in the stator is used both to impart motion to the rotor and to modulate the frictional forces present at the interface. The friction modulation allows bulk motion of the rotor (i.e., for farther than one vibration cycle); without this modulation, ultrasonic motors would fail to operate.
Two different ways are generally available to control the friction along the stator-rotor contact interface.Traveling Wave Vibration and Standing Wave Vibration.Some of the earliest versions of practical motors in the 1970s, by Sashida,for example,used standing-wave vibration in combination with fins placed at an angle to the contact surface to form a motor, albeit one that rotated in a single direction. Later designs by Sashida and researchers at Matsushita, ALPS, and
Canon made use of traveling-wave vibration to obtain bi-directional motion, and found that this arrangement offered better efficiency and less contact interface wear. An exceptionally high-torque 'hybrid transducer' ultrasonic motor uses circumferentially-poled and axially-poled piezoelectric elements together to combine axial and torsional vibration along the contact interface, representing a driving technique that lies somewhere between the standing and traveling-wave driving methods.
A key observation in the study of ultrasonic motors is that the peak vibration that may be induced in structures occurs at a relatively constant vibration velocity regardless of frequency. The vibration velocity is simply the time derivative of the vibration displacement in a structure, and is not (directly) related to the speed of the wave propagation within a structure. Many engineering materials suitable for vibration permit a peak vibration velocity of around 1 m/s. At low frequencies 50 Hz, say a vibration velocity of 1 m/s in a woofer would give displacements of about 10 mm, which is visible to the eye. As the frequency is increased, the displacement decreases, and the acceleration increases. As the vibration becomes inaudible at 20 kHz or so, the vibration displacements are in the tens of micrometers, and motors have been built that operate using 50 MHz surface acoustic wave (SAW) that have vibrations of only a few nanometers in magnitude. Such devices require care in construction to meet the necessary precision to make use of these motions within the stator.
More generally, there are two types of motors, contact and non-contact, the latter of which is rare and requires a working fluid to transmit the ultrasonic vibrations of the stator toward the rotor. Most versions use air, such as some of the earliest versions by Dr. Hu Junhui.Research in this area continues, particularly in near-field acoustic levitation for this sort of application. (This is different from far-field acoustic levitation, which suspends the object at half to several wavelengths away from the vibrating object.)
Canon was one of the pioneers of the ultrasonic motor, and made the "USM" famous in the 1980s by incorporating it into its autofocus lenses for the
Canon EF lens mount. Numerous patents on ultrasonic motors have been filed by
Canon, its chief lens making rival
Nikon, and other industrial concerns since the early 1980s. The ultrasonic motor is now used in many consumer and office electronics requiring precision rotations over long periods of time.
Nikon uses a Silent Wave Motor in many of their lenses, which is an ultrasonic piezo device similar to Canon's USM. It moves things without making noise. The lens focuses silently, handy for snoop photography and for sensitive pets and wildlife.
Sigma HSM (Hyper-Sonic Motor)
Sigma HSM lenses use a motor driven by ultrasonic waves to provide a quiet, highspeed AF. With conventional auto-focusing, auto-focus is driven by electromagnetic force of a DC-motor, gear and shaft mechanism. However, Ultrasonic Oscillation Energy operates the HSM lens and creates high-torque drive. Since this motor has no reduction gears, it has much quicker response to start and stop compared to DC- motor lenses. HSM system has virtually silent auto focus function, very quick responsive auto focus function and "full-time" manual focus, wherein AF can be overridden without disengaging the AF mode altogether.
In-Camera Focus Motor
The in-camera focus motor found on all but the entry level DSLR models (D40, D40X,
D60, D3000, D3100. D5000) is for the older design AF lenses, like the 50mm f/1.8D. These lenses use a mechanical connection and the in-camera motor for focusing. Newer AF-S lenses use an in-lens motor, sometimes called a silent wave motor, hence the “S” tacked onto “AF” that stands for silent.
Nikon is not the only company that produced/produces AF lenses, Tamron, Sigma, Tokina, etc all manufacturer AF lenses. AF lenses will not AF on a camera that does not have an in-camera motor. AF-S lenses will not AF on many older film SLR cameras because the cameras don’t have the necessary electronics, but they will AF on all Nikon DSLR models. AF-S lenses can not utilize the in-camera focus motor – they can only AF using the in-lens motor. The in-camera focus motor is excess baggage if all your lenses are AF-S.
The focusing speed of AF lenses may be significantly faster on professional grade models than on consumer grade models for several reasons. One reason is that professional grade models generally have larger, more powerful, focusing motors that can rotate the focusing mechanism faster. Professional grade models also have better AF systems, currently 51-points as opposed to as few as three and there may be other factors such as the processing speed involved, all of which may mean that the camera and lens recognizes an in-focus condition sooner and does less hunting and therefore quicker focusing.
The in-lens focusing motor’s speed of an AF-S lens is the same regardless of the camera the lens is mounted on; the focusing mechanism can’t turn any faster on a D3 than it does on a D40. However there can be a noticeable difference in focusing speed due to other factors, such as the number of focusing points and the camera’s ability to recognize an in-focus condition – less hunting. My 70-300mm VR lens does less hunting on my D90 than it did on my D50. The time it takes the lens to go from the closest focus to infinity is the same on both cameras, but the lens hunts, going back and forth from close to infinity, much more on the D50.
You'd be surprised how fast some screw-driven lenses are. Most implements are much faster than the canon AFD lenses, particularly on a Dx or Dxxx body that houses a faster screw AF motor than the Dxx
G Series over D Series Nikon Lenses
That absence of the aperture ring on G series lenses means you CANNOT use these lenses on anything other than N65, N80, F100 and newer
Which means that Nikon has finally broken their 40 year tradition of backwards compatability.
Which means the only people who should buy these lenses are people just being introduced to Nikon...