Light rays entering at the edges of spherical lens elements converge at slightly different focal points to light rays entering from the centre. Known as spherical aberration, this optical phenomenon produces soft, low contrast images that look as if
covered with a thin veil. Canon developed aspherical lens elements to counteract this problem. A special non-spherical surface converges central and peripheral light rays at a single focal point to ensure uniform sharpness and clarity over the whole image
area. Now found in almost every EF lens, aspherical lens elements are particularly useful for large-aperture lenses, wide-angle lenses and high quality compact zoom lenses.