A dust reduction system, or dust removal system, is used in several makes of digital cameras to remove dust from the image sensor. Every time lenses are changed, dust may enter the camera body and settle on the image sensor. Digital single-lens reflex cameras (DSLR) are particularly vulnerable to this issue, since the interior of the camera is exposed during lens changes unlike other forms of digital cameras, and the image sensor is fixed, unlike a film camera. Even the tiniest (micrometre-size) dust particles or other contaminants that settle on the face of the image sensor (individual pixels of which have dimensions on the order of ~5 micrometres) may cast shadows and thus become visible in the final image as more or less diffuse grey blobs, depending on aperture. Dust may be generated by internal moving parts or may be moved by air currents within the camera. Some systems remove or clean the sensor by vibrating at a very high frequency—between 100 hertz and 50 kilohertz. Types of systems Different manufactures employ their own version of dust abatement. One type uses a piezo crystal to vibrate a filter which covers the sensor. The second type moves the actual sensor—this may be supplemented by a controlled flow of air. Piezo crystal ultrasonic vibration of filter This system vibrates the thin filter surface that covers the image sensor many tens of thousands of times per second (35,000 to 50,000 hertz) to remove particles from the filter. The system consists of a very thin piece of filter glass placed in front of the image sensor; the area between the filter and the sensor is sealed, so no dust can enter. Whenever the camera is turned on, a piezoelectric driver induces a vibration in the filter glass, shaking dust off. A piece of adhesive located inside the camera traps removed dust. The distance between the filter glass and the sensor also mitigates the problem of dust, since any dust that does adhere to the glass will be held further away from the sensor, and thus produce a larger, more diffuse, and less noticeable shadow. In practice, few Four Thirds system users report having any issues with sensor dust. Olympus invented the system, called a Supersonic Wave Filter (SSWF), and licensed it to Leica and Panasonic. Canon also uses this type of system. Nikon uses a similar system, and they refer to it as high