resonance. The SSWF has been included in all Olympus, Panasonic, and Leica Four Thirds DSLRs, and is often cited as a key advantage of the system by reviewers and users. One disadvantage to the implementation on all current Four Thirds cameras is that the SSWF is triggered whenever the camera is turned on, causing a delay of about 0.8 seconds before the camera is ready to shoot. Sensor shifting This type of system moves the actual sensor to help reduce dust. It vibrates the actual sensor at around 100 Hz. The amount of movement or sensor travel is larger than the higher-frequency filter vibrating types. A crude analogy to compare it to the piezo crystal filter method is something like hitting or banging the sensor to displace the contaminant, whereas the piezo vibrates a filter to make the particles fall off. The sensor may also utilize a negatively charged surface coating to reduce static and help repel negatively charged particles. Konica Minolta is credited with being the original developer of this type of system. Sony and Pentax incorporate sensor shifting in their cameras with dust reduction systems. Need for use with digital cameras These problems are not as critical with film SLRs as the dust disappears as the film is wound on, but with DSLRs the image sensor always remains in the same place. Even with dust particles smaller than 1 micrometre (0.001 mm) and invisible to the human eye, once they land on the image sensor's surface they can degrade the quality of all the images taken thereafter. Furthermore, it can be a difficult task to remove the dust, sometimes making it necessary to send the camera in for servicing. Types of dust particles There are two main types of dust that can potentially degrade image quality: Dust particles that adhere through electric force and dust particles that adhere through intermolecular force. Dust particles adhering through electrostatic charges Most of the contamination to be found on the image sensor surface is caused by dust particles as small as just one micrometre (0.001 mm) adhering to it through electrical charges. The particles themselves carry a positive static electric charge, while the image sensor is negatively charged, which makes them attract each other. The same phenomenon can be observed on the surface of LCD and CRT monitor screens. Dust particles adhering through intermolecular force The intermolecular force is weaker than electrostatic