Walao.. you don't change lens in the middle of an exposure lah!! When not exposing, the shutter and the mirror is down anyway, 2 layers of barrier before it reaches the sensor!! If dust wants to get past those, it would have gotten in when the lens is mounted also!! ;pAs the censor would have a charge (Static) when the power is on, dust particles would have a higher chance to attach onto it. :sweat: