Every time you swap lenses on your digital camera, it's an opportunity for dust to settle on the sensor and make little black specks in your photos. Although many modern cameras have self-cleaning mechanisms that shake dust off the sensor every time you turn on the camera, they may not remove it all. So, it's worth checking your camera once in a while even if you haven't noticed dust in your photos.
How to check for dust
In a dim environment, set the camera to:
I set focus manually so the wall will not be in focus, to blur any texture on the wall and make any specks more apparent. I move the camera during a long exposure for the same reason. I increase contrast to make the specks darker and the wall lighter so the specks are easier to see.
How to clean the sensor
If you find specks that might ruin your photos, you can send the camera to a service center for cleaning, or clean it yourself. I've always cleaned my own. It takes some care, but I use a light touch and haven't had any problems. Here are the products I use, in this order:
Giottos' Rocket Blasters are popular. Remove the lens, expose the sensor, hold the camera facing down (so dust can fall out), and blow a few puffs of air at the sensor to dislodge any dust. If you have a DSLR, you'll need to activate "sensor cleaning mode" to lock the mirror up and the shutter open. This takes care of 50% of my dust. Important safety tip: Do not use canned air, as it can damage your camera.
If the dust is too stubborn to be moved with a blower, this is the next tool to use. This brush spins to create a static charge that attracts dust to the brush. You then lightly brush the sensor from one edge to the other. Do not spin the brush while it's inside the camera. When you're done and you've closed up the camera, spin it again to remove any dust from the brush. This takes care of the next 45%.
For specks that are really stuck. Dab gently at the exact spot where you saw the speck. LensPen sells a kit with a blower and an illuminated loupe that can give you a close-up view of the sensor. Don't wipe the whole sensor with the SensorKlear, as you might smear gummy gunk. I've needed the SensorKlear only once or twice in the past five years.
I've never needed more than this, but if you have streaks or oil on your sensor, you'll need:
A lot of photographers rely on Eclipse liquid and swabs. You moisten a swab, swipe the sensor, then dry the sensor by swiping lightly with one or two clean swabs. I avoid wet cleaning unless it's absolutely necessary, because in the past other products have left me with streaks that were hard to remove. Current products might work better, though.
These are essentially the same tools the service centers use. In all cases, follow the instructions exactly. If your camera has In-Body Image Stabilization (IBIS), the sensor floats on a delicate mechaniism, so be very gentle.
Questions are welcome.