I have a couple lenses that don't have an internal focusing system, and whenever I want to put them back in my bag, I turn the focus ring to make the lens shorter.
I noticed that when the lens is set to auto focus, the focus ring is harder to move than when it is on manual focus. Can I damage the focusing system by turning the focus ring when the lens is set to auto focus?
I would think that if the lens is off the camera it wouldn't cause any damage. Am I right?
I think that depends a bit on the construction of the AF mechanism in the lens. If there is resistance when turning the focus ring, this also means that there is greater force applied to move the mechanics, and so there is greater stress in the material. I would personally switch over to manual focus for that procedure.
As a rule of thumb, don't turn anything that has a motor engaged, it doesn't matter if it's your camera lens or your garage door.
Put the lens in manual mode to disengage the motor, then turn the focus ring all you want.
This depends on the lens. Focus can be overridden manually for Canon lenses with ring USM designation and Nikon lenses that have MA/M (manual-auto/manual) switch. As already mentioned, if there is any resistance/specific noise when turning the focus ring, you probably shouldn't be doing this.
You should avoid turning the focus motor by hand. The motor itself will probably take it, but the transmission from the motor to the focus ring is where you put the strain.
If there is a resistance when turning the focus motor by hand, it means that there is a gear system for reducing the high speed of the motor to the low speed of the focus ring. When the motor is pulling the gears, there is little resistance as it's always a smaller gear pulling a larger. When you turn the motor by hand, there is a lot more resistance as it's always a larger gear pulling a smaller one.
I think your intuition is correct.
When the lens is mounted and set to autofocus, turning the focus ring turns all of the autofocus mechanisms back to the in-camera motor, and backwards - the gears and stuff are designed to bring the high-speed rotation coming out of the electric motor down to something more reasonable for manipulating the lens, and driving this setup backwards applies stress it wasn't designed to handle. You can feel this - as you say, it's harder to move the focus ring when autofocus is on. So, don't do it when the lens is mounted.
I think the same rule of thumb applies when the lens is not mounted. If turning the focus ring is noticeably harder when autofocus is on, don't do it. Otherwise, feel free.
Many lenses (e.g. Nikkor AF-S) are specifically designed to let you do this, so then it's no problem obviously.
My camera (Olympus E-510) changes the focus of the lens to make the lens as short as possible when I turn the camera off. So I made a habit of turning the camera off before removing the lens. If the lens doesn't return to its shortest length for some reason (when the camera goes into power saving mode it seems to forget it has a lens), I simply turn it on and off again.
All the Olympus lenses I have are focus-by-wire. Turning the focus ring when the camera is off doesn't do anything. So I don't really have a choice. Pushing the inner lens element to make the lens shorter doesn't seem like a good idea.
