Reading glasses, often referred to as “readers,” are a common solution for the difficulty many people experience when focusing on close-up objects as they age. This requirement for near vision correction is nearly universal, making the numbers on an eyeglass prescription a frequent source of confusion. Understanding the specific terminology and measurements on a reading glasses prescription is the first step toward clear and comfortable vision, helping in selecting the most appropriate corrective lenses.
The Underlying Need: What Is Presbyopia?
The condition that necessitates reading glasses is called presbyopia, a gradual loss of the eyes’ ability to focus on nearby objects. This physiological change is a normal part of aging, typically becoming noticeable around the early to mid-40s and progressing until about age 65. The primary mechanism is the hardening and loss of flexibility of the eye’s natural lens, located just behind the iris.
In younger eyes, the lens is supple and changes shape easily with the help of the surrounding ciliary muscle to focus light onto the retina for near tasks. As the lens ages, protein cross-linking causes it to become more rigid (sclerosis). This hardening means the ciliary muscles can no longer easily alter the lens’s shape to achieve the necessary refractive power for close-up vision. Consequently, light from near objects focuses behind the retina, resulting in blurred text or difficulty with fine detail work.
Deciphering the Reading Glasses Prescription
The specific power needed for reading is noted on a prescription as “ADD,” which stands for Addition power or near addition. This is the amount of extra magnifying power, measured in Diopters (D), that must be added to a person’s distance correction to achieve clear near vision. The ADD value is always positive (+) because it represents a converging lens power.
The range for the ADD power typically begins around +0.75 D and increases gradually as presbyopia progresses, often maxing out around +3.00 D to +4.00 D. This value is generally the same for both the right eye (OD) and the left eye (OS) because the underlying physiological cause is symmetric. For individuals who wear glasses for distance vision, the ADD power is combined with the Sphere (SPH) power to calculate the final reading prescription.
The SPH column indicates the lens power needed to correct for nearsightedness (-) or farsightedness (+) for distance viewing. If a person only requires reading glasses, the SPH value might be 0.00. In a dedicated reading-only prescription, the SPH value will be the total positive power required, such as +2.00.
Custom Prescription Versus Over-the-Counter Readers
The primary distinction between a custom prescription and over-the-counter (OTC) readers lies in the level of personalization. OTC readers are a mass-produced, one-size-fits-all solution, offering uniform spherical magnification identical in both lenses. These readers are available in fixed steps (e.g., +1.00, +1.25) and do not account for individual variations.
A custom prescription addresses elements that OTC readers cannot correct. It allows for monocular correction, providing a different power for each eye if needed. A custom prescription also includes correction for astigmatism, a common refractive error caused by an irregularly shaped cornea. Astigmatism correction uses the Cylinder (CYL) value for lens power and the Axis value (1 to 180 degrees) for orientation.
Custom lenses can also incorporate prism correction if there are issues with eye alignment, a feature absent from generic readers. While OTC readers offer temporary relief for mild presbyopia, a custom prescription ensures clear, balanced vision that minimizes eye strain. Only a prescription lens can combine the necessary ADD power with the precise SPH, CYL, and Axis corrections.
Alternative Lens Designs for Reading
Beyond standard single-vision reading glasses, several alternative lens designs integrate reading power. The oldest form of multifocal correction is the bifocal lens, which features a distinct, visible line separating the distance vision portion from the reading portion at the bottom. The reading segment often has a flat-top shape, providing a wide field of view for near work, and the clear division makes it simple to switch between viewing distances.
A more advanced option is the Progressive Addition Lens (PAL), commonly known as “no-line bifocals.” PALs offer a seamless, gradient transition of power from distance vision at the top, through an intermediate zone, and into the full reading power at the bottom. This gradual change provides clear vision at all distances without the visible line found in bifocals.
For people who spend significant time at a computer, specialized occupational or “office” lenses are available. These designs are optimized for intermediate and near viewing distances, offering a wider and more comfortable field of vision for screen work than the smaller intermediate zone of a standard PAL.