What’s the difference between ultrasonic motor and ring-type autofocus, and how can you tell which is which? The answer is surprisingly simple. Below we tell you everything you need to know.
Motor-based ultrasonic autofocus uses a small motor in the lens that drives the mechanism via gearwheels.
It’s usually not particularly fast but is generally quieter in operation than lenses that have a standard electric motor, or where autofocus is driven by a motor in the camera body.
Ring-type ultrasonic autofocus is based on two large rings that are fitted inside the lens. There’s no motor or any gears as such, and the system is generally faster as well as being practically silent in operation.
In some lenses, such as range-topping 70-200mm f/2.8 telephoto zooms, ring-type autofocus can be so quick that it’s almost instant.
Unfortunately, most companies like Canon, Nikon and Sigma apply USM, AF-S and HSM designations respectively to lenses that have either motor-based or ring-type ultrasonic autofocus.
- Don’t know these acronyms? Click here to find out what your lens markings mean
It’s therefore very difficult to know which is which without thoroughly checking through all the specifications before you buy.
If you can try out the lens in a shop before you make your choice, you’ll generally find that if the focus ring doesn’t rotate during autofocus, then it’s a ring-type ultrasonic system, although this isn’t always the case.
Ring-type ultrasonic ‘rear-focus’ systems don’t have to move the comparatively large front element of the lens. This helps to boost speed.
Autofocus speed will be reduced when using a teleconverter with compatible lenses, and if the widest effective aperture with the teleconverter fitted shrinks below f/5.6 autofocus usually becomes unavailable.
DSLR Lenses: 7 questions photographers must ask about their next piece of glass
How to find your lens’ sweet spot
Lens distortion: what every photographer must know
10 common camera mistakes every photographer makes