How to clean and maintain your camera and lenses
When removing/attaching the lens, dust or debris may get inside the camera and adhere to the surface of the image sensor. If this happens, it may appear on the final image as a dark spot.
Your camera (SLT-A55/A55V/A33) is equipped with the anti-dust vibration function to prevent dust from adhering to the image sensor. If dust or debris remains, however, clean the image sensor by following the below procedure.