-Focal length: Light rays which are parallel to the axis of the lens, after passing through a convex lens meet at a point on the other side of the lens. This point is called as a focus of the lens. The distance between focus and the optical centre (or node) of the lens is called as focal length of that lens. Focal length of a camera lens may also be defined as the minimum practicable distance between film/image sensor and (the optical centre of) the lens. Focal length of a camera lens is usually marked in mm on the front of the lens. Focal length remains fixed for a given lens (other than zoom lens). SLR camera may take lenses having different focal lengths.