Far Point of Clear Vision

FAR POINT OF CLEAR VISION (punctum remotum), the point at which the eye is set with complete accommodation at rest, i.e., the point from which rays entering the relaxed eye converge on the light-sensitive layer of the retina and form a clear image upon it. The distance (r) of the far point from the cornea can be expressed in linear units or in diopters (D). In the latter case, it determines the refraction (R) of the eye. For a normally constructed eye (emmetropia), the far point is so distant that rays coming from it to the eye are very close to being parallel. For such an eye, r = infinity meters and R = 0. Strictly speaking, for an emmetrope, objects located several meters beyond 5 m produce on the retina an image just as distinct as very distant objects, because when an object is brought from a very distant point to 5 m from the eye, its image shifts backward by exactly the thickness of the light-sensitive layer (by 0.06 mm). From this moment, the eye needs the work of the accommodative apparatus to place the image within the zone of the sensory elements. In nearsightedness (myopia), the far point has a positive finite value and is located in front of the eye; in farsightedness (hypermetropia), it is behind the eye and is expressed by a finite negative value. For methods of determining the far point - see Refraction.