Ametropia Optics PDF

**Optics of Ametropia** **(Refractive error)** -Ametropic eye fail to focus parallel rays on the retina -F2 of the eye dose not fall on the retina Refractive error of the eye: -Myopia -Hypermetropia -Astigmatism Causes of refractive errors: 1-Curvature 2-Axial 3-Indecial 4-Change in the...

**Optics of Ametropia** **(Refractive error)** -Ametropic eye fail to focus parallel rays on the retina -F2 of the eye dose not fall on the retina Refractive error of the eye: -Myopia -Hypermetropia -Astigmatism Causes of refractive errors: 1-Curvature 2-Axial 3-Indecial 4-Change in the position of the refracting media 5-Absence of one of the refracting media Myopia: -F2 in front of the retina -Axial (big eye) -Curvature (steep cornea) -Index (Nucleo-scerosis, Cortical hydration) -Change in position (Forward displacement of lens According to the age of onset and progression Myopia is classified to: -Early childhood myopia -Simple myopia  -Degenerative or malignant myopia Hypermetropia: -F2 behind the retina -Axial (small eye) -Curvature (flat cornea) -Change in position (backward displacement of lens According to ciliary muscle stimulation hypermetropia is classified to: -Manifest hypermetropia: strongest convex lens accepted for clear distant vision -Latent hypermetropia: masked by ciliary tone -Facultative hypermetropia: can be overcome by accommodation -Absolute hypermetropia: in excess of the amplitude of accommodation As we name the lens by its F2, the eye is named by its far point. The far point: The position of the object at which after refraction by the eye its image formed on the retina, in the relaxed eye The point of meeting of the rays emerge from the eye, the retina as object -The retina and far points are conjugate points Myopia: -The far point in front of the eye, nearer than infinity (-Ve) Hypermetropia: -The far point behind the retina (+Ve) **Astigmatism:** the refractive power of the eye varies in different meridians -Regular astigmatism: -Two refracting meridians -At right angle to each other -The change from one meridian to the other is smooth and regular -Sturm's conoid Simple hyperm., simple myopic, mixed, compound myopic,compound hypermetropic. -With the role astigmatism: Vertical meridian more curved than the horizontal -Against the role astigmatism: Horizontal meridian more curved than the vertical **Correction of refractive errors:** Aim: Parallel rays of light to focus on the retina To chose a lens positioned at P, its F2 must coincide with the far point of the eye F = 1/f = 1/r F = power of correcting lens f = focal length of correcting lens r =The distance of the FP from P **Effective power of the lens:** -The correcting lenses are not placed at P -Placed at spectacle plan -Back vertex distance: distance from back surface of the lens to the anterior surface of the cornea In hypermetropia, as the lens moved forward, the imag moves also forward, so a weak correcting power is needed. i.e. the effective power increases In myopia, as the lens moved forward, the image moves forward, so stronger correcting power is needed. i.e. the effective power decreases  **Effective power formula:** F2 = 1/ f1 - d = F1 / 1 - df1 F1 = Power of first correcting lens F2 = Power of second correcting lens f1 = focal length of first correcting lens d = displacement of correcting lens in meter **Spectacle magnification:** optical correction of ametropia is associated with change in the retinal image size Spectacle magnification= corrected image size / uncorrected image size Relative spectacle magnification (RSM): corrected ametropic image size / emmetropic image size **Axial ametropia:** 1-spectecal at F1; RSM = 1.00  2- Spectacle nearer than F1: RSM \> 1.00 in myopia RSM \< 1.00 in hypermetropia 3- Spectacle further than F1: RSM \> 1 in hypermetropia RSM \< 1 in myopia **Refractive ametropia:** The image size change regardless the relation to F1 RSM \>1 in hypermetropia and \< 1 in myopia In corrected refractive ametropia; the closer the correcting lens to the eye, the less change in retinal image size