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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?