When you receive an eye prescription, it often contains complex numbers and symbols. These notations are precise instructions for creating corrective lenses tailored to your vision needs. Understanding the meaning behind the common minus (-) and plus (+) signs helps clarify how your glasses or contact lenses will improve your sight.
Understanding the Minus Sign
A minus sign (-) on an eye prescription indicates myopia, commonly known as nearsightedness. Individuals with nearsightedness experience clear vision for objects close to them, but distant objects appear blurry or out of focus. This occurs because the eye’s shape causes light rays from distant objects to converge and focus in front of the retina, rather than directly on its surface. The retina is the light-sensitive tissue at the back of the eye responsible for sending visual information to the brain.
To correct nearsightedness, a negative lens power is prescribed. These lenses are concave, meaning they are thinner in the center and thicker at the edges. Their function is to diverge, or spread out, the incoming light rays before they enter the eye. This divergence allows the eye’s natural focusing mechanisms to then converge the light precisely onto the retina, bringing distant images into clear focus.
Understanding the Plus Sign
Conversely, a plus sign (+) on an eye prescription signifies hyperopia, commonly referred to as farsightedness. People with farsightedness find it challenging to see near objects clearly. This visual difficulty arises when the eye’s structure causes light rays entering the eye to focus behind the retina.
A positive lens power is used to correct farsightedness. These lenses are convex, characterized by being thicker in the center and thinner at the edges. Their role is to converge the light rays before they reach the eye. This pre-convergence helps ensure that once the light passes through the eye’s natural lens, it lands directly on the retina, enabling clear vision for close-up tasks and reducing overall eye strain.
The Significance of the Numbers
The numbers that follow the plus or minus signs on an eye prescription, such as -2.50 or +1.75, represent the lens power in units called diopters. A diopter quantifies the optical power of a lens and indicates how much correction is needed to focus light accurately onto the retina. The magnitude of this number, regardless of whether it is positive or negative, reflects the degree of vision correction required.
A higher numerical value, for example, -5.00 compared to -1.00, or +4.00 versus +1.00, indicates a stronger prescription. The further the diopter value is from zero, the greater the refractive error and the more powerful the corrective lens needs to be to achieve clear vision.