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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 :-).

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Which camera are you using? I can't find anything in a Canon DSLR manual which advises turning the camera off to change lenses. –  Matt Grum Dec 31 '10 at 9:19
    
@Matt Grum: Like many users I've encountered on this site, I have my gear on my profile. I'm using a Nikon D7000. The message is on page 26 of this online manual: nikonusa.com/pdf/manuals/noprint/D7000_ENnoprint.pdf. –  Tom Dec 31 '10 at 17:24
    
Hmm seems to be a Nikon thing. What does it say under manual sensor cleaning? Are you supposed to turn the camera off, remove the lens, turn it back on, open the shutter and then clean the sensor? –  Matt Grum Dec 31 '10 at 18:51
    
Ha, yes! Page 286 of the same manual: turn the camera off, remove lens, turn it back on... –  Tom Dec 31 '10 at 19:01
    
My guess: Nikon doesn't test for this, so they tell you not to do it. –  wberry Jul 10 '13 at 4:15

6 Answers 6

up vote 28 down vote accepted

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.

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To add further weight to the assumption that they all do this... if they didn't ensure the power was disconnected first, and/or didn't ensure that power didn't get applied to the data pins, there'd be hundreds of thousands of dead lenses/cameras around and product recalls to fix it. That sort of thing wouldn't just sometimes kill a lens — power applied to a data connection (which isn't specifically protected for such a circumstance) will kill the electronics almost instantly. –  drfrogsplat Dec 31 '10 at 7:23
    
I like the answer overall. Clearly everyone else does too, so I am going to go ahead and accept this. But do you buy into the dust argument? I also find it strange that says Canon says to do this and Nikon says not to. @Matt Grum seems to think that Nikon is just covering themselves for no real reason. –  Tom Jan 1 '11 at 17:35
    
By the way, I was wondering if "frying" the lens or camera could happen -- or if this was really just about dust. You guessed right since I'm basically a student :-). –  Tom Jan 1 '11 at 17:37
    
Thanks! I'm afraid the only comment I can add to the 'dust question' is an anecdotal one... I've been using dSLR cameras for... ohhh... a decade or more now, and have never bothered turning off my camera while changing lenses... In all that time I've never lost a single picture to dust... I'll go one step further- I can't recall ever having had to erase a dust spot in Photoshop. All that seems to indicate (to me at least) that the problem of the charged sensor attracting dust- even if it is completely true- is a very minor problem at best. –  Jay Lance Photography Jan 1 '11 at 19:57

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.

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I've often heard the "charged sensor" argument but never seen evidence to back it up and I'm afraid I don't buy it. When the camera is on but not exposing a frame there is nothing going on on the sensor to generate any charge. And if there were the shutter would still be closed and any charge would not be strong enough to attract dust from outside the camera body, about 4cm away. Yes changing lenses could stir up the air inside the camera, but no more (and probably less) than the mirror flipping up while shooting! –  Matt Grum Dec 31 '10 at 9:11
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Also with Canons (and assume other brands as well) sensor cleaning is performed with the camera on. The manual advices activating the sensor cleaning function and blowing the sensor to clean it. If the sensor were charged there's no way this would work. –  Matt Grum Dec 31 '10 at 9:25
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Why does my explanation seem contrary to common sense? Common sense says to me that when the sensor is not doing anything, there's no current flowing through it and thus no charge. –  Matt Grum Jan 2 '11 at 14:39
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@Itai Your wife can hear the TV on mute because the amplifier is still trying to amplify the input signal even if the TV is not sending a signal - the noise on the line gets amplified. A camera is not a mains powered device, keeping the sensor powered on all the time seems like a bad idea in a battery powered device. As far as I know the only thing you need to do before shooting is earth the charge from the photosites before opening the shutter, something that is done before any shot anyway... –  Matt Grum Jan 3 '11 at 2:57
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@lindes Think about it this way, if the sensor cleaning mode somehow kept the sensor in an uncharged state, why does the camera not keep the sensor in this state all the time, since keeping the sensor uncharged ought to drain the battery less... –  Matt Grum Jan 3 '11 at 3:00

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.

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+1 for the airline analogy :-)... I hate that. –  Tom Jan 1 '11 at 17:31

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.

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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.

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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.

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