A method for determining whether or not a transparent protective cover of a video camera comprising a lens-based optical imaging system is partly covered by a foreign object is disclosed. The method comprises:



obtaining (402) a first captured image frame captured by the video camera with a first depth of field;
obtaining (404) a second captured image frame captured by the video camera with a second depth of field which differs from the first depth of field; and
determining (406) whether or not the protective cover is partly covered by the foreign object by analysing whether or not the first and second captured image frames are affected by presence of the foreign object on the protective cover such that the difference between the first depth of field and the second depth of field results in a difference in a luminance pattern of corresponding pixels of a first image frame and a second image frame. The first image frame is based on the first captured image frame and the second image frame is based on the second captured image frame.