Sony ILCE-7 Instruction Manual page 36

Attaching a lens
Notes 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 converts the light to
an electric signal), 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
landing on the image sensor. However, always make sure to quickly change
the lens away from dusty locations when attaching/removing a lens.
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