The Canon EF-S 18-135mm f/3.5-5.6 IS USM has a designation of Nano USM on the lens. What does this mean? What are the advantages of having it vs STM or USM?
We have a terminology thread that usually covers these questions but this is not yet addressed in it and I'm interested in the comparisons to the similar technology that Canon offers.
Canon autofocus lenses have long focused on speed and not so much on smooth and quiet operation. This is what we know today as USM (Ultrasonic motor). As usage of DSLRs for video became more popular, Canon introduced STM (Stepping motor) lenses that provided a smoother experience; albeit at a slight disadvantage on the speed side.
Nano USM addresses the shortcomings of STM, provides the speed of USM, and has the smoothness of STM. The performance of Nano USM is based on the similar ultrasonic vibration converted into movement methodology that USM has employed for some time. The actual operation of the autofocus is nearly silent which is important to mainly video but can be a consideration under certain still image shooting scenarios.
Its claims end up posing it as the best of both worlds; meaning that both still image shooters and videographers will likely be pleased with the operation and results. Since this technology is available in only Canon lenses, and the first one was only announced recently - only time will tell if it's performance is as good as claimed.
For more information, see the Canon USA article: What's New: Nano USM - A New Ultrasonic Motor Technology
One feature not mentioned above is the ability of the new lens to connect to the Canon Power Zoom Adapter PZ-E1. As it was explained to me, this adapter allows videographers the opportunity to zoom in or out while shooting a video scene using a hand-held remote, rather than a manual zoom. Probably not important if you're only shooting stills, but could be very handy while shooting video.
Canon's Nano USM focusing motors have so far been used in the EF 18-135mm f/3.5-5.6 IS USM and EF 70-300mm f/4-5.6 IS II USM lenses.
It is a type of AF motor that, like Ring USM, uses micro-piezo vibrations to move the lens focus elements. In the case of the Nano USM AF motors, the movement is in a linear, rather than rotational, direction.
For a more detailed look with plenty of pictures of the new Nano USM AF motor, check out Roger Cicala's blog entry from his teardown of an EF 70-300mm f/4-5.6 IS USM II.
The use of direct linear movements of the AF elements means Nano USM lenses can only be focussed manually using a focus-by-wire¹, rather than mechanical, connection between the lens' focusing ring and the focusing elements.
This is similar to Canon's Stepping Motor (STM) technology that also requires manual focus-by-wire¹ (although Canon's stepping motors use rotational movements). This eliminates the ability to focus the lens when using "dumb" extension tubes or freelensing. You can't move the focus elements to park them when the lens is not attached to a powered up EOS camera body either. The camera must be turned on and metering active in order to move the lens' focusing elements. There are even a very few (mostly older, discontinued) USM lenses that are focus-by-wire, but the vast majority of USM lenses are capable of manual focus adjustment by moving the focus ring without any electrical connection or power.
There are some considerations to be made when shooting with the focus-by-wire manual focus system of the STM and Nano USM lenses that may or may not be of concern depending on the user. For more information on the comparative differences of three different types of AF motors in three lower priced Canon 50mm lenses please see: What would be a better lens, 40 or 50mm prime, for walk around?
¹ "focus-by-wire" means there is no direct link between the manual focus ring and the lens' focus mechanism. Rather, the focus ring transmits a set of electronic instructions to the camera body which in turn sends electronic instructions to the lens to move the focus elements using the same motor as when the camera is focusing the lens automatically. The smallest amount of focus movement possible with an STM AF motor is one "step" of the stepping motor in the lens. It is possible, however, to manually focus (even if in "steps" by wire) without turning off AF via the switch on the side of the lens barrel. This is known as "full time manual focus". To do this you must first half press the shutter button or press the AF-ON button or AE Lock (*) button to activate the camera's focusing system before you can manually focus the lens.
