The number “1.75” frequently seen on over-the-counter reading glasses is not a formal prescription but rather a straightforward measure of the lens’s magnifying power. This value represents a specific level of vision correction designed to compensate for the natural changes that occur in the aging eye, providing clear vision for close-up tasks.
Defining the Diopter Strength
The number 1.75 on reading glasses refers to the Diopter (D), the standard measure of a lens’s optical power. This unit indicates the degree to which a lens converges or diverges light, directly influencing where the eye is able to focus. A strength of +1.75 D means the lenses are designed to focus light at a specific close distance, making small print or nearby objects appear clear.
The plus sign (+) signifies a converging lens, which is thicker in the center and magnifies the image. This positive power is necessary for near vision correction because it aids the eye’s natural focusing mechanism, helping to bring objects held at a typical reading distance into sharp focus. The diopter strength is derived from the inverse of the focal length, meaning a higher diopter number corresponds to a shorter focal length and a stronger magnifying effect.
Reading glass strengths typically increase in quarter-diopter increments, such as +1.50, +1.75, and +2.00. The +1.75 D strength represents a moderate level of magnification, providing a noticeable improvement for those struggling with small text and focusing difficulty for near work.
Presbyopia and the Need for Reading Glasses
The underlying biological reason for needing a +1.75 strength is a condition known as presbyopia, the gradual, age-related loss of the eye’s ability to focus on nearby objects. This change is a normal part of the aging process where the crystalline lens inside the eye becomes less flexible and harder over time. The surrounding muscle also loses some effectiveness, making it difficult to adjust focus quickly between far and near distances.
A common symptom of presbyopia is the need to hold reading material farther away to see the text clearly, a phenomenon often called “long-arm syndrome.” The required correction strength increases as the lens’s flexibility continues to diminish. While this varies greatly among individuals, a strength around +1.75 D often becomes necessary for comfortable reading as people move into their late 40s or early 50s.
For a person with otherwise normal distance vision, this +1.75 D magnification compensates for the eye’s diminished natural focusing power, allowing for clear vision at a normal reading distance of about 14 to 16 inches. This need for corrective power progresses over time, which is why people often find they need to increase their reading glass strength every few years until the process stabilizes, typically around their early 60s.
Over-the-Counter Labels Versus Formal Prescriptions
When purchasing reading glasses off the shelf, the +1.75 label represents the total lens power for near vision correction. This simple label is distinct from a formal prescription written by an eye care professional, which contains multiple values to address all of an individual’s visual needs. In a comprehensive prescription, the +1.75 power is equivalent to the “ADD” (Addition) power listed in the near vision section.
The ADD power is the extra magnification required to correct for presbyopia and is added to the distance prescription. Over-the-counter readers assume the distance prescription is zero, meaning they only provide the ADD power of +1.75, which is the same in both eyes. A formal prescription, however, also includes values like “SPH” (Sphere) for distance vision correction and “CYL” (Cylinder) to correct for astigmatism, which simple readers cannot address.
Prescription reading glasses are customized to account for any difference in vision between the two eyes, as well as the presence of astigmatism. Over-the-counter options provide a one-size-fits-all solution with the same power in both lenses, which is sufficient for many people who only have presbyopia.
Determining If +1.75 Is the Correct Strength
To verify if +1.75 D is the appropriate strength, an individual can use a standardized reading chart test, which is often available in stores that sell reading glasses. This self-testing method involves reading text of various sizes held at a comfortable distance, typically 14 to 16 inches, to find the lowest magnification that allows for clear, comfortable reading. It is advisable to choose the lowest power that provides clear vision without strain.
Signs that the +1.75 strength is too weak include continued eye strain, headaches after reading, or still needing to hold the book far away to achieve focus. Conversely, if the strength is too high, text may appear clear but you may experience blurriness or dizziness when trying to look up and across the room. The strength is correct when reading material is held at a natural, comfortable distance and appears sharp with no associated discomfort.
Even if over-the-counter readers at +1.75 D feel adequate, it remains important to have regular, comprehensive eye examinations. These exams not only determine the precise corrective power needed for each eye but also check for other underlying eye health issues such as glaucoma or cataracts that simple reading glasses do not correct. Eye care professionals recommend exams every two to four years for adults between ages 40 and 54.