Anatomy & Refractive States of the Eye
Emmatropia and ametropia

As mentioned earlier, the goal of the transparent structures of the eye is to bring rays of light to a sharp focus on the retina. To take that a little further it could be said their goal is to bring rays of light to a sharp focus on the retina when the gaze is directed at optical infinity. Optical infinity is a distance of about twenty feet. At that distance rays of light are considered to be traveling parallel to each other. When a patient is viewing an object at optical infinity, 20 feet or more, accommodation is at rest. This means, the crystalline lens is as thin as it can get manifesting its minimum power.

Emmetropia is illustrated in on page 10. When a person is emmetropic, it means that when he is gazing at optical infinity, the rays of light which enter the eye come to a perfect focus on the retina. And in this case, no correction is needed for distance viewing. The distance correction therefore is plano, or zero. However, if that same person happened to be presbyopic, he or she would need corrective lenses for near vision activities such as reading. Near vision viewing requires additional power, so if the crystalline lens does not have sufficient accommodative power corrective lenses will be needed. This situation, that of the emmetropic presbyope is common, especially among early presbyopes.

The other conditions, illustrated on pages 10-11, are examples of the various ammetropias. And to emphasize once again, all these conditions are defined as occurring when accommodation is at rest. If we look at the illustration of myopia, we can see the rays of light coming to focus in front of the retina. The commonly used term for myopia is “nearsighted.” Myopic patients can be corrected through the use of minus powered lenses. Hyperopia or “farsightedness” is just the opposite. Here, the rays of light come to focus behind the retina and this condition is corrected through the use of plus powered lenses. So in the case of either myopia or hyperopia, when the gaze is directed at optical infinity and accommodation is relaxed, there is no clear point of focus on the retina and vision is blurred. The greater the magnitude of the myopia or hyperopia, the greater the blur.