Understanding your vision prescription can be confusing due to its symbols and numbers. The plus (+) and minus (-) signs on an eyeglass or contact lens prescription indicate the type of vision correction needed. This distinction is fundamental to understanding whether your eyes are nearsighted or farsighted.
Decoding Your Prescription: Plus vs. Minus
A plus sign (+) on your prescription indicates that you are farsighted, a condition medically known as hyperopia. Conversely, a minus sign (-) signifies nearsightedness, or myopia. These numbers, measured in units called diopters, quantify the strength of the lens required to correct your vision. The further the number is from zero, regardless of the plus or minus sign, the stronger the prescription needed.
Positive lenses, also called convex lenses, are thicker in the middle and cause incoming light rays to converge or bend inward. This convergence helps to bring light into focus on the retina for individuals with farsightedness. Negative lenses, or concave lenses, are thinner in the middle and cause light rays to diverge or spread out. This divergence is necessary to correctly focus light onto the retina for those with nearsightedness.
Understanding Nearsightedness
Nearsightedness means distant objects appear blurry, while close-up objects remain clear. This occurs because the eye focuses light in front of the retina, rather than directly on its surface. The primary optical reason is an eyeball that is too long from front to back, or an excessively curved cornea.
When the eyeball is elongated, light rays converge too soon, causing the focal point to fall short of the retina. This results in a blurred image for faraway objects. Symptoms include squinting to see clearly at a distance, eye strain, and headaches. This condition develops during childhood and adolescence, stabilizing in early adulthood.
Understanding Farsightedness
Farsightedness is a refractive error where distant objects are seen clearly, but close-up objects appear blurry. This condition arises because the eye focuses incoming light rays behind the retina. This improper focus is due to an eyeball that is shorter than average or a cornea with insufficient curvature.
When the eye’s length is too short or the cornea is too flat, light entering the eye does not converge enough by the time it reaches the retina, causing the focal point to land beyond it. Symptoms can include difficulty with close-up tasks, eye strain, and headaches, particularly after reading or other detailed near work.