An eye prescription’s abbreviations and numbers can be confusing, especially the term “ADD.” This value is essential for people needing vision correction for both distance and close-up tasks. Understanding the meaning of Addition power, and what a value of 2.25 signifies, clarifies a significant aspect of modern vision correction.
Defining the “ADD” Parameter
The term “ADD” on an eye prescription stands for “Addition” power. This value is a supplementary lens strength, measured in diopters (D), incorporated into the lens to boost near vision clarity. It represents the extra magnifying power needed to help the eye focus on objects held at a close reading distance, such as a book or phone screen.
This number is always a positive (+) value, even if the plus sign is not explicitly printed on the prescription. The Addition power is designed to work in conjunction with the distance prescription, which corrects for far vision, to create a final total power for near tasks. The ADD value is typically the same for both the right eye (OD) and the left eye (OS).
The Vision Condition Requiring ADD
The need for an Addition power stems from a common, age-related condition known as presbyopia. This is a natural change in the eye that affects nearly everyone, typically beginning to cause noticeable symptoms around the age of 40. Presbyopia is not an eye disease but rather a gradual hardening of the crystalline lens inside the eye.
The lens loses the natural flexibility necessary for the eye to change its focus from far objects to near objects, a process called accommodation. When the lens becomes stiffer, the eye struggles to pull close-up text into sharp focus, causing people to hold reading material farther away. The ADD power compensates for this diminished ability to accommodate, restoring clear vision for reading and other close work.
How the ADD Value Creates Reading Correction
The Addition power of 2.25 is the extra power that must be added to the distance prescription, not the final reading prescription itself. To determine the total near vision correction, the ADD power is algebraically combined with the Sphere (SPH) value of the distance prescription. For instance, if a person’s distance Sphere power is -1.00 (nearsighted) and the ADD is +2.25, the total reading power becomes +1.25 (-1.00 + 2.25).
This combined power is placed in the lower section of the spectacle lens to provide the near correction. The method of incorporating this power varies depending on the lens type chosen. Bifocal lenses provide the ADD power in a distinct segment at the bottom, creating a visible line separating the distance and near powers.
Progressive Addition Lenses (PALs) seamlessly integrate the ADD power, gradually increasing magnification down the lens without a visible line. This design allows for a smooth transition from distance viewing at the top, through an intermediate range (like a computer screen), to the full reading power at the bottom. Trifocals include three distinct zones to cover distance, intermediate, and near vision needs.
Understanding the Strength Scale of the ADD Number
The typical range for Addition power prescriptions begins around +0.75 and rarely exceeds +3.00 to +3.50, covering the vast majority of presbyopia cases. The value of +2.25 is considered a moderate-to-strong correction within this scale.
The required ADD power increases with age as the eye’s natural lens continues to lose flexibility. A +2.25 ADD typically corresponds to the level of presbyopia seen in people in their late 50s or early 60s. For example, one study found the average required ADD power for individuals aged 55 to 64 to be approximately 2.03 diopters.
A +2.25 value reflects an advanced stage of the eye’s natural aging process, suggesting a strong need for magnification to see clearly at close range. The progression of this power tends to slow down after age 50, meaning the change from +2.00 to +2.25 may take longer than earlier increases seen in the 40s.