1) The position at which rays of light from a lens converge to form a clear and sharply defined image on a focal plane. 2) The action of adjusting the distance between the lens and subject to make light rays converge to form a clear and sharply defined image of the subject.Generally, the act of adjusting a lens to produce a sharp image. In a camera, this is effected by moving the lens bodily towards or away from the film or by moving the front part of the lens towards or away from the rear part, thus altering its focal length.