The manual warns that you should turn the camera off to remove the lens, but it doesn't say why.
I suspect this may have to do with dust, but I could imagine there are lots of other reasons. I'm hoping someone can tell me the reason for doing this, so it will help me to never forget :-).
When I get this question from my students, the underlying question is often based on the persistent myth that taking a lens off without first turning off the camera will 'fry' the lens, the camera, or both (depending on who you talk to).
With both Canon and Nikon cameras the leftmost pins on the body (when looking at the camera) are the VBat (6 volt lens power) pins, so when the lens is disengaged and rotated counterclockwise the first thing that happens is that power is removed from the lens. Since none of the other lens contacts sweep over the VBat pin, that scenario is actually physically impossible. Additionally, since the lens power is disengaged from the moment the lens is rotated, the lens is no longer able to pass information back through the communication (dcl and dlc) and clock (dclk) pins. Thus there is very little danger of bad data getting passed between the camera and the lens and somehow 'corrupting' anything unless you really snap the rotation of the lens and there "happened" to be communication going on at just the right moment. In any case, simply turning off the camera and turning it back on would clear such an error.
While I have no direct experience with the pin and contact patterns of other brands of SLR/dSLR, I would be very surprised if the others (Sony, Pentax, etc.) did it differently.
I've been changing lenses with the camera on as long as I've been using DSLRs and I know a lot of other photographers who do the same. If there was a significant risk of causing damage the manufacturers would have had it in the neck by now as so many pros do it.
I suspect this is another case of there being an astronomically small risk, but the camera manufacturers have nothing to lose by telling you to turn the camera off. It's the same as when the airlines tell you to turn off your Nintendo DS during takeoff/landing. You'd severely hope it takes more than a pocket games console to down an airliner, but instead of proving this, it's easier for them to just ban it.
The main reason to switch lenses while the camera is off it to avoid exposing the sensor to dust while it is charged as that increases the chances of dust sticking to it. It is really not a big deal but if you want to stack the odds in your favor, you know what to do.
There is probably an infinitely small chance that turning the lens will cause a lens read error since the lens contacts move while the lens is turned to remove it.
I change lenses with the camera on from time to time, but I also always run the dust removal function on the camera afterwards. Now, that may be camera brand specific (Pentax for me), so I don't want to assume it generally applies. The only time I absolutely make sure it is off is when I put a manual lens on because the Pentax SR function will ask for the focal length.
I have never bothered to turn the camera off when switching lenses.
If I ever had any problems like a lens not working properly, it would easily have been fixed by turning the camera off and on again, and it can't have happened many times because I can't recall any incident. Besides, that could just as well have happened if I turned the camera off for switching the lens.
As Matt Grum pointed out that he couldn't find any instruction to turn the camera off in the manual, I checked the camera manuals for my EOS 300 and my EOS 5D mark II, and the procedure for attaching a lens doesn't even mention turning off the camera. I checked the manual for a lens also, and that doesn't mention it either.
I definitely remember reading it somewhere, so it might be there in some manuals, but if there was a good reason for turning the camera off, it would be in every manual, both for cameras and lenses, and it would be hard to miss.
As the comments to the original question indicate, it depends on exactly which camera you are talking about.
Nikon lenses have a mechanical connection for aperture control that is spring loaded at both the camera end and inside the lens that could suffer damage if disconnected while in the wrong position. If the tab on the control lever in the camera is bent it causes incorrect aperture settings that usually result in overexposure (because the tab usually bends up and the lens doesn't stop down as far as the camera tells it to stop down).
Some Nikon cameras also have mechanical connections between camera and lens for auto focus. If the lens is disconnected while the focus screw is moving I would imagine the potential to damage it would be much like you can strip the head of a screw by turning an improperly applied screwdriver.
Canon EOS lenses have no mechanical connections to the camera (other than the mount itself) and there are no warnings against removing lenses while the camera is turned on in Canon User Manuals. All control connections between an EOS camera and lens are electronic and the main power supply coming from the camera is the first connection to disengage when the lens is turned to dismount it. The connection is also designed so that the camera's power pin doesn't brush against any of the lens' other pins as the lens is rotated off.
I don't have enough knowledge of manufacturers' systems, other than Nikon and Canon, to comment on them, but if the connection is all electrical it should be safe to remove the lens without turning off the camera. If the connection includes some mechanical linkages it may not be.
I've never bothered to turn off my camera before removing a lens, and so far I haven't broken one because of it. I have had dusty cameras, but I mostly suspect it might cause some electrical glitches somewhere, which is why they want you to turn it off.
I think the main reason is to "park" (hard disk terminology) the components. e.g. VC/IS elements can by their nature move around. When you power the camera off, those elements are "parked". I'm pretty certain I've read in manuals/leaflets (yes it's a unsavoury habit I've picked up, I do read manuals..) that come with lenses.
This may also be true with focus elements.
Interestingly this Nikon Australia NPS article about transporting lenses says that you should turn off VR, while the camera is on if you're going to ship a lens.
Here's the guts of the post:
When transporting lenses, either by road or air please ensure you do the following:
- Ensure both the Front and Rear lens caps are affixed correctly.
- Turn your VR off (where applicable) whilst the lens is still connected to a camera that is powered on, this will ensure the correct locking of the VR mechanism so that it does not move excessively whilst in transit.
- Ensure the lens is well protected, whether in a camera case or if freighting, the correct amount of packing material is used.
Also in some scenarios, with some lenses it is possible that the rear element may protrude slightly (one of the reasons teleconvertors are not recommended with some lenses) into the camera body increasing the risk of a knock as it is removed.
The manual for my camera (a Pentax K100D) states that you should turn the camera off to prevent possible damage to the autofocus motor: if you bump the shutter-release button while changing the lens, it's possible that the now-exposed autofocus drive shaft will catch on something.
My 400mm telephoto works on my D90 ONLY if I put it on with the camera turned "ON". If I connect the lens first and then turn on the camera, I get an error. I would have to pay a couple hundred dollars to have the chip re-flashed to work on the D90 without having to have the camera on first.
