There are two filters inside the camera and directly in front of the sensor. One is a UV/IR filter, the other is a "low pass" filter (it reduces the problem of moiré in an image). Anyway, the camera uses the piezoelectric effect to vibrate the front filter in the hope that this will "shake" any dust particles loose. This will not always works and sometimes more aggressive cleaning is needed.
A hand-squeezed air blower (e.g. a Giottos "Rocket" blower) might be able to blow off dust that the vibration can't clear. Exceptionally soft-bristled paint brushes are sometimes used (pristinely clean, of course) and there are even versions of these with a grounding wire so they can discharge any static cling that might be responsible for dust sticking to a sensor.
There are other tools... gummy "Gel" sticks is one I've seen recently. If the simple vibration and puffs of air don't clear dust, then I usually resort to wet-cleaning using "Eclipse" cleaning solution (a few drops) on a "Sensor Swab". Eclipse is nearly pure methanol so it evaporates rapidly and leaves no residue behind.
From time to time, dust will manage to get onto a sensor and, from time to time, the self-clean wont be enough to clear it. So it's an inevitability of DSLR photography that the sensor will, just occosionally, require a manual cleaning.
Tim Campbell
5D III, 5D IV, 60Da
