Progressive lenses correct vision at multiple distances without the visible lines found in bifocal or trifocal lenses. They offer a seamless transition of power, allowing the wearer to shift focus naturally from far away to reading distance. Understanding the specific numbers and abbreviations on the prescription slip is the first step in demystifying this lens technology.
Decoding the Standard Prescription Fields
A standard eyeglass prescription uses specific abbreviations to detail the required refractive correction for each eye. The prescription is structured with separate rows for the right eye (OD, or Oculus Dexter) and the left eye (OS, or Oculus Sinister). The term OU (Oculus Uterque) indicates a measurement that applies to both eyes.
The SPH (Sphere) value specifies the main lens power needed to correct for nearsightedness or farsightedness, measured in diopters (D). A minus sign (-) signifies correction for nearsightedness, while a plus sign (+) indicates correction for farsightedness. This SPH number represents the primary power for the distance viewing zone at the top of the progressive lens.
The CYL (Cylinder) value addresses astigmatism, which is an imperfection in the curvature of the eye’s cornea or lens. If this column is blank, correction is not required. If a CYL value is present, a corresponding AXIS number must also be included, which is a degree between 1 and 180. The AXIS indicates the exact orientation on the lens where the astigmatism correction must be placed.
These SPH, CYL, and AXIS values define the distance correction, which is the foundational part of the progressive lens design. This portion is used when looking straight ahead, such as when driving. All subsequent power calculations for intermediate and near vision are built upon this initial distance prescription.
The Crucial Element: Understanding the ADD Power
The defining characteristic of a progressive lens prescription is the “Addition” (ADD) power, which is a single, positive number, usually the same for both eyes. The ADD power represents the extra magnification needed to compensate for presbyopia, the age-related loss of the eye’s ability to focus on nearby objects. This value is measured in diopters and typically ranges from +0.75 to +3.00, increasing as the eye’s natural focusing ability diminishes over time.
This single ADD number is mathematically added to the SPH power to determine the final reading prescription at the bottom of the lens. For example, if the distance SPH is -1.00 D and the ADD power is +2.00 D, the near vision power will be +1.00 D. The progressive lens design creates a smooth, continuous increase in power from the distance SPH to the final near vision power.
As the ADD power increases, the complexity of the progressive lens design also increases. Higher ADD values require a greater change in power over a short vertical distance on the lens. This often results in a narrower, more focused visual corridor. This is a trade-off managed by manufacturers to provide clear near vision while minimizing peripheral distortion.
Measurements Essential for Progressive Lens Fitting
Unlike single-vision lenses, progressive lenses require highly precise physical measurements to ensure the different power zones align correctly with the wearer’s eyes. The most important measurement is the Pupillary Distance (PD), which is the distance between the centers of the two pupils, measured in millimeters. For progressive lenses, a dual PD is often taken, measuring the distance from the nose bridge to the center of each pupil separately (monocular PD).
The PD positions the distance viewing zone of the lens directly in front of the pupil, ensuring the wearer looks through the correct optical center. A second measurement, the Fitting Height (or Optical Center Height), is equally important. This value determines the vertical position on the lens where the progressive corridor begins, marking the precise point where the power transitions from distance to intermediate.
This height is measured from the center of the pupil down to the lowest point of the frame. Its accuracy is critical for a successful fit. If the fitting height is incorrect, the wearer may have to tilt their head uncomfortably to access the reading zone, or they may inadvertently look through the intermediate zone for distance vision. Many modern progressive lenses have a minimum fitting height, often around 14 to 18 millimeters, which dictates which frames are compatible with the design.
How Progressive Lenses Function
The prescription numbers are translated into the physical lens structure, which is divided into three distinct zones of vision. The top portion of the lens is the Distance Zone, used for viewing objects far away. The bottom portion incorporates the full ADD power, creating the Near/Reading Zone for close-up tasks like reading a book or smartphone.
Connecting these two primary zones is the Intermediate Zone, often called the progressive corridor. This corridor is a narrow, vertical pathway that gradually increases in power, providing clear vision for middle distances, such as working at a computer. This seamless, invisible change in power is achieved through a complex curvature on the lens surface.
Because the lens surface is mathematically altered to provide this continuous change in power, a trade-off is introduced in the peripheral areas of the lens. This optical consequence is experienced as peripheral distortion or a “swimming” sensation when quickly moving the eyes to the side. The specific design aims to minimize this peripheral blur while maximizing the width and clarity of the three main viewing zones.