News » Emmetropia And Ametropia
It is said that eye ammetrope where the rays come from a virtually infinite distance will be in focus on the retina at the maximum negative accommodation conditions: the retina is located exactly at the level of primary eye focus. In all other cases, the eye is ametrope. There are three kinds of ametropia: myopia, hyperopia and astigmatism. MYOPIA. The refracted rays are on fire in front of the retina; diotro eye is equivalent to a positive lens too strong compared to the length of the eye; the retina is located behind the main fire back of the eye. This condition requires: a) when the power of the cornea or the crystalline lens is higher than normal; b) When the lens is too close to the cornea and the anterior chamber is, therefore, lower than normal; c) When the diameter anto-back of the eye is higher than normal. HYPEROPIA The refracted rays are focused behind the retina; eye diotro equivalent to a weak positive lens with respect to the length of the eye; the retina is located in front of the main fire back of the eye. This condition requires: a) when the power of the cornea or the lens is lower than normal; b) when the lens is too far away from the cornea, that is when the anterior chamber is deeper than the norm; c) when the diameter anto-back of the eye is lower than normal; d) when the lens is absent (aphakia). ASTIGMATISM The refracted rays do not gather at one point but form a fan of Sturm; The eyepiece diopter is equivalent to a combination hemisphere-cylindrical. According to the position that the focal lines of the fan of Sturm occupy compared to the retina, the astigmatism can be simple, compound or mixture. a) ASTIGMATISM SIMPLE. One of the two focal lines falls on the retina while the other is located in front (myopic simple astigmatism) or behind it (simple hypermetropic astigmatism. b) ASTIGMATISM COMPOUND. None of the two focal lines falls on the retina; both are located or in front of (compound myopic astigmatism) or behind it (hyperopic astigmatism compound). c) MIXED ASTIGMATISM. One of the two focal lines falls in front and the other behind the retina.
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