the present invention provides extended depth of focus to human eyes by modifying contact lenses , interocular implants , or the surface of the eye itself . this is accomplished by applying selected phase variations to the optical element in question . the phase variations edf - code the wavefront and cause the optical transfer function to remain essentially constant within some range away from the in - focus position . this provides a coded image on the retina . the human brain decodes this coded image , resulting in an in - focus image over an increased depth of focus .