lens....................

focal length of a lens is constant. it does not change. if light beam comes from infinite, it will be focused on the focal plane. the distance between the focal plane to the lens is the focal length.

if light is 打橫咁入. it will be focused on the centre of focal plane (F). if it is incidenced at an angle to the centre line. it will focused off centre on the same focal plane. the focal length is still the same.

pls refer to the below diagram:
http://trueforce.com/images/Optics/convex2.jpg