Sony ILCA-99M2 - Manual (Page 56)

When shooting, remove the front lens cap from the front of the lens.
Mount the lens by aligning the two orange index marks (mounting indexes) on the
lens and camera.
Hold the camera with the lens mount facing down to prevent dust and debris from
entering the camera.
2
While pushing the lens lightly toward the camera, turn the lens slowly in the
direction of the arrow until it clicks into the locked position. 3