Basic Refraction & Lensometry

Emmetropia is the state where parallel light rays focus precisely on the retina without accommodation. Myopia (nearsightedness) occurs when the eye is too long or the cornea too steep — light focuses anterior to the retina; corrected with minus (concave) lenses. Hyperopia (farsightedness) occurs when the eye is too short or cornea too flat — light focuses behind the retina; corrected with plus (convex) lenses. Astigmatism occurs when the cornea or lens has unequal curvature in different meridians, creating two focal points; corrected with cylindrical lenses. Presbyopia is the age-related loss of accommodation, typically becoming symptomatic around age 40.