Why can some focus rings turn endlessly, and how is that different from zooming?

Question

I understand focal length as being related to the distance between the lens and the sensor, and longer focal lengths often mean physically longer lenses. That made me wonder how focusing works: does the lens move in and out to focus, and if so, doesn’t that change the focal length? How is this handled in prime lenses versus zoom lenses?

I’ve also noticed that when I turn a zoom ring, I can often see internal lens groups moving and there are clear end stops. But on some lenses, the focus ring seems to turn endlessly. Past a certain point it no longer changes focus, yet I can keep rotating it. Why are some focus rings designed this way?

user15871 · Answer

Although designs vary on a lens by lens basis, lenses that allow you to endlessly move the focus ring use a design that allows the focus ring to slip when the end of travel of the lens' focus element is reached. This is most often seen in lenses that use a ring type focus motor that drives the focus element using very high frequency electrical pulses rather than a direct mechanical connection.

"The piezoelectric motor with progressive wave" section here (at the very end of a rather long article) explains in great detail the way such a focus motor works. Nikon refers to such a focus motor as a Silent Wave Motor (SWM), Canon calls it Ultra Sonic Motor, Sigma refers to it as Hyper Sonic Motor (HSM).

Why can some focus rings turn endlessly, and how is that different from zooming?

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