Page 7 - Practical-Refraction-English

B Ametropia Emmetropia, Ametropia An eye which is not correctly focussed (that is, where the image of a distant object is not formed on the retina of the unaccommodated eye) is said to have a refractive error or an ametropia (from the Greek meaning disproportionate eye). 1 The different types of ametropia are classified into three categories: myopia, hypermetropia and astigmatism. 1) Myopia (Shortsightedness): 2) Hypermetropia (Longsightedness): Myopia is the state of refractive error in which the image Hypermetropia (or hyperopia) is the state of refractive of an object located at infinity is formed by the eye (in error in which the image of an object located at infinity its relaxed state) in front of the retina. The word myopia is formed by the eye (in its relaxed state) behind the reti- comes from the Latin myops and Greek muôps meaning na. The word hypermetropia comes from the Greek a person who narrows the eyes (“squints” or peers). The hyper = beyond (measurement) and ops = eye. person with uncorrected myopia sees distant objects as blurred but can see close objects clearly (they are “sigh- Optically, the hypermetropic eye presents a lack of ted” at “short” range). power relative to its length. This may be classified either as being because it is too short relative to its power Optically, the myopic eye presents an excess of power (axial hypermetropia (the majority of cases for hyper- Practical Refraction relative to its length. This may be classified either as metropia in excess of 5.00D)), or because it is insuffi- being because it is too long relative to its power (axial ciently powerful relative to its length (refractive hyper- myopia (the majority of cases for myopia in excess of metropia). 5.00D)), or because the eye is too powerful relative to its length (refractive myopia). Hypermetropia is corrected by the introduction of a plus (positive) powered lens so as to move the image forward Myopia is corrected by the introduction of a minus and reposition it onto the retina. (negative) powered lens, so as to move the image back and reposition it onto the retina. © Essilor International © Essilor International © Essilor International © Essilor International Figure 3: The Myopic Eye and the Principle of its Figure 4: The Hypermetropic Eye and the Principle of Correction its Correction 7 Copyright © 2008 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute.