Sony SLT-A35 Instruction Manual page 29

Note on changing the lens
When changing the lens, if dust or debris gets inside the camera and
adheres to the surface of the image sensor (the part that acts as the film), it
may appear as dark spots on the image, depending on the shooting
environment.
The camera is equipped with an anti-dust function to prevent dust from
adhering to the image sensor. However, quickly change the lens away from
dusty locations when attaching/removing a lens.
If dust or debris adheres to the image sensor
Clean the image sensor using [Cleaning Mode] in the Setup menu
(page 37).
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