What is the Usm motor ?
What are the advantages?
In ideal cases almost instant autofocus operation, very precise focusing, silent operation.
How is it works ?
In simple ultrasonic energy transformed into rotational energy by piezo cristals to move the lens focusing group. What are the piezo cristals? For example quartz (SiO2). These cristals makes movements because of electric energy or when pushing them electric energy is made. Interestingly this motors has lower battery consumption than the “normal” screw driven motors.
Lots of Canon lenses equipped with Usm motor.
Canon 135 f2, 200 f2.8, 24-70 f2.8, 24-105 f4, 300 f2.8, 300 f4, 400 f5.6, 500 f4, 600 f4, 800 f5.6, 70-300/f4-5.6, 18-55 Usm, 60/2.8 Macro,100 f2.8 macro, 100 f2.8 macro, 85/1.8 just to name a few, these lenses feauture the “real ring type” Usm motor. The 50/1.4 has not this real ring type Usm motor, just the name is Usm, Canon called Micro Usm. The fastness of this motors despite the name are not equal. The most speedy usm motors can be found in the “galactic long primes”, the 85 f1.8, 135 f2, 24-105 f4 and 200 f2.8 lenses.
The properly working usm motor is a valuable feature for example in wedding and wildlife photography.
Other manufacturers also produce solutions similar to Canon’s, according to my understanding the Canon USM solution is the best.
Non Canon similar solutions
Nikon has their Swm (Silent wave motor), Sigma has an HSM(Hypersonic motor), Sony has their (SSM) Super sonic motor, Pentax SDM Supersonic drive motor, Olympus SWD (Supersonic wave drive), Panasonic Xsm (Exra silent motor).
Is the difference big compare to the normal screw driven lenses?
I would say the difference in usage is quite big between a good Usm autofocusing lens and a “normal” screw driven lens. For wide angle lenses the Usm motor is not essential, but for telephotos where the depth of field can be very shallow, is a very useful feature. Also helps to track quickly moving subjects, such as animals, birds, vehicles, etc. The precision are much better in Usm lenses.
If I imagine my wonderful Canon 200 f2.8 L with a normal focusing mechanism it would diminished the value of the lens a lot. No matter how good is your lens optically if you cannot make a shot, or make a shot misfocused.
In another way if the lens has Usm drive, doesn’t mean anything how good is the lens optically.