If you own more than one lens, be careful when you switch to the other lens! Try to work quickly. The longer you have your lens off your camera, the more time there is for dust to drift into the camera body, and that dust eventually can work its way onto the sensor. Sensor dust then becomes visible in your images!
If at all possible, keep your camera pointed downward as you change lenses, so gravity is at least working in your favor, pulling dust away from your camera. Also shield your camera from wind if you absolutely have to change lenses on a windy day. Watch out, though. Just shielding your camera from the wind doesn’t magically mean the air is clean and dust free.
The absolute worst place to change lenses is at the beach. That sandy salty humid air is just bound to get junk inside your camera and eventually onto your sensor. I would say avoid doing that at all costs.
I’ve seen some people claim that it is important to turn off your camera before you change lenses. I have checked with numerous experts who all say it is NOT an issue to worry about. You don’t have to turn off your camera to change lenses. If you have any concerns about your specific camera, check your user manual. I don’t think it’s a problem, though, at least from what I have been told.
Oh another point. If you ever sit the camera or lens on the ground, you’ll get dirt and dust on the bottom of the camera or lens, making it even more likely to get dust inside your camera and eventually on the sensor. You’re better off sitting the camera on a table, a bench, a chair, concrete, a rock, anything but on dirt!
And for the lens you put away in your camera bag, make sure you put a CAP on the back! Never just put a lens in your camera bag without a cap on the back camera mount. That will just get dust inside the lens and the next time you mount that lens on a camera, you’ll just transfer the dust into your camera!
What does sensor dust do to an image? I shows up in your photographs as little dark spots, sometimes very tiny, sometimes fairly big. How crisp and well-defined the spots are depends on what aperture you are using. The higher the f/stop, the more well-defined the sensor dust appears to be. At lower f/stops, the dust is more soft.
Here’s a waterfall photo that has sensor dust on it.
I have circled some of the sensor dust I noticed. And here is an enlarged section so you can see it more clearly.
THAT is what you are trying to avoid by exercising care when changing lenses!
Eventually the sensor dust becomes enough of a problem that you will have to get your sensor cleaned. You could take your camera to a local camera store, or send it off to your camera manufacturer for cleaning. You can also buy special swabs and other tools for cleaning the sensor yourself. Exercise caution when doing that. You really need to research that carefully to ensure you don’t damage your camera. I am not saying you shouldn’t clean your own sensor. I do it all the time. Just make sure you know what you are doing before you try it!
At least do what you can by reducing the dust that gets into your camera in the first place. Start by practicing good lens-changing habits.
Happy Lens Changing,