My understanding of focal length is that it is the distance from the lens to the sensor; in my observations: higher focal lengths make for longer lens.

Then I started thinking about how a lens focuses. If my previous notion of focal length was true, how does focus work? Does the lens move in or out to focus? Doesn't that also change the focal length? What about in a prime lens?

When I rotate the zoom ring I can clearly see the inner lenses moving back and forth. Also, there is a clear start and stop point where I can't rotate the ring any further.

When I rotate the focus ring I can't notice the lenses moving at all unless it's just so tiny that it's hard to see. Also, I can rotate my focus ring endlessly; although, it doesn't seem to have an effect past certain points. Let's say I go 1 turn past focus on my subject, it takes 1 turn to get back in focus. If I go 30 turns past focus, it still only takes 1 turn back to get back in focus. So if focus if the focus ring only actually does something within a certain range, why does the design let you turn it endlessly?


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    Hi Zuko, welcome to Photo SE. You've actually asked more than one question here. From what I can see you want to know (a) how is focal length defined, and does it change when focussing? (b) why does the focus ring allow you to keep turning even when the focusing mechanism no longer moves? For the first question I refer you to photo.stackexchange.com/questions/16549/… - the second part of your question doesn't seem to have been answered so I'd edit your question to focus on that (pardon the pun) and you should get some answers. – NickM Aug 23 '15 at 15:23

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).

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    This is great, but what actually happens with the lenses that makes it focus? What's a "focus element" and what does it do to focus? – Kenmore Aug 24 '15 at 16:50

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