The terminology on an eyeglass prescription can seem like a confusing code. When you encounter the notation “ADD 1.50,” you are looking at a specific measurement related to your eye’s ability to focus on close-up objects. This value prescribes the precise amount of magnifying power needed for tasks like reading or using a smartphone. It is a crucial component for people who need different lens strengths for different viewing distances.
Deciphering the “ADD” Term
The abbreviation “ADD” stands for “Addition” or “Added Power.” It refers to the extra magnifying strength incorporated into the lens to achieve clear near vision. This power is always a positive value (a plus lens) designed to converge light rays and provide magnification. The measurement is given in diopters (D), the unit used to express optical power.
The numerical value of \(1.50\) signifies an addition of \(1.50\) diopters of magnifying power. Addition values typically range from \(+0.75\) to \(+3.00\) diopters, making \(+1.50\) a moderate strength. This supplemental power is either added to the distance prescription to create a multi-focal lens or used alone for simple reading glasses. The ADD power compensates for the eye’s natural focusing decline, allowing comfortable viewing at a standard reading distance of around 16 inches.
The Biological Reason for Addition Power
The need for addition power is caused by presbyopia, a natural, age-related condition. Presbyopia is the gradual loss of the eye’s ability to change focus from far to near objects, a process called accommodation. This focusing mechanism relies on the flexibility of the eye’s crystalline lens and the surrounding ciliary muscle.
As a person ages, the crystalline lens hardens and loses elasticity. This stiffening prevents the lens from changing shape sufficiently to focus on close-up items. Because the eye can no longer generate its own magnifying power, the \(1.50\) diopter addition must be provided externally by the corrective lens. This physiological change is a normal part of aging, typically beginning around age 40.
How Addition Power is Delivered
The \(+1.50\) addition power is a measurement incorporated into different lens designs based on the wearer’s needs and existing distance prescription. For those who only need reading correction and have clear distance vision, the ADD power creates single-vision reading glasses. In this design, the \(1.50\) diopter strength is applied consistently across the entire lens surface.
If the person also wears glasses for distance vision, the addition power is integrated into a multi-focal lens. Two common types are bifocals and Progressive Addition Lenses (PALs). Bifocal lenses have a distinct line separating the distance prescription area from the lower segment, which contains the \(1.50\) ADD power for near work.
Progressive Addition Lenses offer a seamless transition from the distance power at the top of the lens to the full \(1.50\) ADD power in the lower portion. This gradient allows for clear vision at all ranges, including intermediate distances like a computer screen, without the visible line of a bifocal. The choice of lens type determines how the \(1.50\) power is structured, but the prescribed strength remains the same.