TY - CHAP M1 - Book, Section TI - Refractive Errors A1 - Duncan, Jacque L. A1 - Parikh, Neeti B. A1 - Seitzman, Gerami D. A2 - Papadakis, Maxine A. A2 - McPhee, Stephen J. A2 - Rabow, Michael W. A2 - McQuaid, Kenneth R. PY - 2022 T2 - Current Medical Diagnosis & Treatment 2022 AB - Refractive error is the most common cause of reduced clarity of vision (visual acuity). In the normal state (emmetropia) objects at infinity are seen clearly. Focusing on objects nearer than infinity requires an increase in the refractive power of the crystalline lens by the process of accommodation. In farsightedness (hyperopia) objects at infinity are not seen clearly unless accommodation is used and near objects may not be seen because accommodative capacity is finite. Farsightedness is corrected with plus (convex) lenses. In nearsightedness (myopia) the unaccommodated eye focuses on objects closer than infinity. The markedly nearsighted eye is able to focus on very near objects without glasses. Objects beyond this distance cannot be seen without the aid of corrective (minus, concave) lenses. Astigmatism, in which the refractive errors in the horizontal and vertical axes differ, is corrected with cylindrical lenses. Presbyopia is the natural loss of accommodative capacity with age. Persons with emmetropia usually notice inability to focus on objects at a normal reading distance at about age 45. Farsighted individuals experience symptoms at an earlier age. Presbyopia is corrected with plus lenses for near work. SN - PB - McGraw-Hill Education CY - New York, NY Y2 - 2024/03/28 UR - accessmedicine.mhmedical.com/content.aspx?aid=1184191990 ER -