Chapter-05 Errors of Refraction

BOOK TITLE: Basic Ophthalmology

1. Jogi Renu
Publishing Year
Author Affiliations
1. MGM Medical College, Indore (MP), Pt. Jawahar Lal Nehru Memorial Medical College, Raipur, Chhattisgarh, India, India, Mahatma Gandhi Memorial Medical College, Indore, Madhya Pradesh, India, Pandit Jawahar Lal Nehru Memorial Medical College, Raipur, Chhattisgarh, India
Chapter keywords


Cornea and lens are the focusing elements of the eye. The average power of normal emmetropic eye is +58 to + 60D. Errors of refraction( ametropia) can occur due to abnormalities in axial length of eyeball, curvature of the cornea or lens, refractive index of the media or position of the lens. Myopia (short sight) may be congenital, simple or pathological. It may be treated by spherical concave lenses, contact lenses or by surgery. Hypermetropia (far sight) may be latent or manifest and is corrected by spherical convex lenses or contact lenses. Astigmatism may be irregular or regular which may again be according to rule or against the rule. The bundle of rays refracted from a regular astigmatic surface is called the “Stum’s conoid”. Astigmatism is treated with cylindrical lenses. Aphakia or absence of lens may be corrected by spectacles, contact lens or intraocular lens implantation. Presbyopia is an age related insufficiency of accommodation and is corrected by convex spherical lenses. Radial keratotomy (RK), photorefractive keratectomy (PRK) and laser assisted in situ keratomileusis (LASIK) are few of the refractive surgical procedures. Refractive error of the eye may be determined objectively by retinoscopy or by auto-refractometer and subjectively by the postmydriatic test.

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