Emmetropia

The parallel rays coming from infinity are diverted (refracted) from the system cornea/crystalline on a specific portion of the retina that allows the sharp vision: the macula. So an eye that can perfectly converge parallel rays coming from infinity on the macula is an eye defect-free and is said emmetropic (see video). In this eye has gone a harmonious and balanced development between its anteroposterior and its refractive system that is, the cornea and lens. In Italy, by custom, an emmetropic eye has a’visual acuity (or visual acuity) of 10/10 (ten / tenths) (Snellen fraction). They are often confused the tenths of this fraction, then how many lines that express the law on patient’ottotipo (the table using the ophthalmologists) out of a total of 10 (then the patient will read; 1/10; 2/10 and so on up to 10/10 for the smallest letters) with diopters (improperly degrees) that are, the unit of measure of the power of the lenses (positive, negative or cylindrical) necessary to correct a particular defect of refraction . (For example, a slight myopic needs an eyewear - 2 diopters to see the 10/10 otherwise your eyesight without glasses would be below 1/10).
Nature has endowed the human eye (and many animal species) the ability to maintain focus the image of an object when it is approaching (that is, the condition of automatically varying the focus). This possibility is called accommodation and is implemented by the modification of the crystalline that keeps images sharp that reach the retina placed at different distances from this. Emmetropic eye then, that is, one whose rays are perfectly focused on the macula and the 'accommodation only intervenes in near vision. In the process of accommodating the crystalline lens can vary its form (from spherical to lenticular, thus changing its refractive power) by the contraction of a small muscle ring (the ciliary muscle) contracting that determines the increase of the thickness of the lens. The increase of the radii of curvature of the anterior surface and posterior lens allows divergent rays, that originate from near, to be focused on the macula. This mechanism is so fast that the brain does not become aware until he appears with age, the loss of elasticity of the lens (Presbyopia), or if the eye presents a significant degree of hypermetropia.