When reviewing an eyeglass prescription, many people encounter the acronym PAL. This term is shorthand for Progressive Addition Lens, designating a specific type of multifocal lens design. Unlike single-vision lenses that correct for just one distance, PALs are engineered to provide clear vision across a range of viewing distances within a single pair of glasses. Understanding what a PAL is and how it functions can help demystify your prescription.
Defining Progressive Addition Lenses (PAL)
Progressive Addition Lenses are a modern solution for presbyopia, an age-related condition involving the gradual loss of the eye’s ability to focus on nearby objects. This loss occurs when the natural lens inside the eye becomes less flexible. PALs address this by incorporating a smooth, gradient change in lens power from the top of the lens to the bottom.
The advantage of PALs over traditional bifocal or trifocal lenses is the absence of visible lines on the lens surface. Traditional lined multifocals create an “image jump” when the eye crosses the demarcation line. Progressive lenses eliminate this disruption, offering a seamless transition that is both smooth and cosmetically appealing. This design allows the wearer to look through different points on the lens to achieve sharp focus at virtually any distance.
How PAL Corrects Vision Across Distances
The design of a Progressive Addition Lens is structured into three distinct zones of vision, each addressing a specific viewing distance. The upper portion of the lens contains the prescription for distance vision, used for activities like driving or viewing scenery. This zone holds the lowest magnifying power, allowing clear focus on objects far away.
Moving down the lens, a narrow vertical corridor, often called the progression corridor, provides the intermediate vision power. This zone is calibrated for middle-range tasks, such as looking at a computer screen or reading a menu held at arm’s length. The power within this corridor increases steadily until it reaches the maximum magnification required for near vision.
The lowest section of the lens is the near vision zone, which contains the full “Addition” power needed for up-close activities like reading or examining a smartphone. A side effect inherent to this complex curvature is peripheral distortion, or unwanted astigmatism, on the sides of the lens. This area of blur can cause a slight “swim” effect when the wearer quickly moves their head or eyes.
Interpreting the PAL Measurements and Adaptation
A measurement associated with a PAL prescription is the “ADD” power, which stands for Addition power. This number represents the extra magnification, measured in diopters, added to the distance prescription to achieve clear near vision. The ADD value is typically the same for both the right eye (OD) and the left eye (OS) because presbyopia affects both eyes equally, so it is often written only once on the prescription form.
The ADD power range generally falls between +0.50 and +3.50 diopters, and this value directly influences the optical design of the lens. As the required ADD power increases, the width of the clear intermediate and near zones tends to become narrower, while peripheral distortion areas expand. Proper frame selection and fitting are necessary to ensure the lens is positioned correctly, allowing the eye to comfortably access all three zones.
Adaptation to PALs requires a conscious effort, often taking up to two weeks for the brain to fully adjust to the new visual experience. Instead of moving only the eyes, wearers must learn to point their nose and head toward the object they wish to focus on, using the appropriate part of the lens. For tasks like reading, the wearer must look down through the lower portion of the lens, and for distance, they must look straight ahead through the top.