Myopia, commonly known as nearsightedness, is a widespread refractive error that results in distant objects appearing blurred while close-up vision remains clear. This blurring occurs because the eye focuses incoming light incorrectly, causing the image to form in front of the retina. The goal of vision correction is to redirect these light rays so they precisely land on the retina, restoring clear distance vision.
Identifying the Corrective Lens
The specific type of lens used to correct myopia is known as a concave lens. It is also referred to as a diverging lens or a minus lens due to its optical effect. Physically, a concave lens is thinner in the center and thicker toward its outer edges.
This shape is engineered to diverge the light rays before they enter the eye. The lens compensates for the eye’s tendency to focus light too strongly, which is the underlying issue in myopia. A concave lens effectively reduces the overall focusing power of the visual system, allowing the image to be formed correctly on the retinal surface.
The Optical Principle of Correction
Myopia typically arises because the eyeball is slightly too long from front to back, a condition known as axial myopia, or sometimes because the cornea and natural lens have an overly steep curvature, leading to excessive focusing power. In either case, the light rays entering the eye converge too quickly, causing the focal point of the image to fall short of the retina.
The corrective concave lens is placed in front of the myopic eye to counteract this excessive convergence. As parallel light rays from a distant object first pass through the concave lens, they are spread apart. This initial divergence reduces the overall refractive power of the optical system before the light reaches the eye’s natural focusing components.
The diverged light rays then continue through the cornea and the crystalline lens of the eye. This two-step process results in the light converging at a point farther back than it would naturally. The ultimate effect is that the focal point is successfully pushed back to land directly upon the retina, producing a sharp, clear image.
Delivery Methods and Prescription Strength
The corrective concave lens is most commonly delivered to the eye either through eyeglasses or contact lenses. Eyeglasses place the lens slightly away from the eye’s surface, held within a frame, and are often the first choice for vision correction due to their ease of use and immediate tolerability. Contact lenses, conversely, are thin plastic devices worn directly on the surface of the cornea, which can offer a wider field of view and less peripheral distortion compared to glasses.
The power, or strength, of the concave lens required to achieve clear vision is measured in units called diopters (D). This unit quantifies the optical power needed to correct the refractive error. Since the lens must diverge light to correct myopia, the prescription value is always denoted by a negative, or minus, sign.
A prescription of -2.00 D, for example, indicates a need for a concave lens with a power of two diopters to push the focal point back onto the retina. A numerically larger negative value, such as -6.00 D, signifies a stronger degree of myopia and requires a lens with greater diverging power. The strength of the prescription is precisely calculated to match the specific distance the light is currently focusing in front of the retina.