Progressive lenses are designed to provide clear vision at all distances—far, intermediate, and near—within a single pair of glasses. A single-vision prescription, by contrast, provides optical clarity for only one specific focal length, such as only for distance viewing or only for reading. Converting a progressive prescription simply involves isolating the optical power intended for a single, desired distance from the multi-functional lens formula.
Understanding the Components of a Progressive Prescription
A progressive lens prescription contains several distinct measurements necessary for its manufacture. The foundation of the lens power is represented by the Sphere (SPH), which corrects for nearsightedness or farsightedness, measured in diopters. A negative sign indicates correction for distance (myopia), while a positive sign suggests a need for a converging lens (hyperopia). These SPH values typically represent the power needed for clear distance vision.
The prescription also includes the Cylinder (CYL) and Axis values, which are necessary to correct astigmatism. The CYL value specifies the magnitude of astigmatism correction, while the Axis, measured in degrees from 1 to 180, denotes the orientation of that correction. These three values—SPH, CYL, and Axis—form the complete optical profile for the distance portion of the lens.
The final, distinguishing component is the Addition (ADD) power, which represents the extra magnifying power required for close-up tasks. This ADD power is a uniform value applied across both eyes and is the power boost needed to compensate for presbyopia, the age-related loss of focusing ability. This value is algebraically combined with the distance correction to create the near-vision portion of the progressive lens.
Calculating the Single Vision Distance Prescription
Deriving the single-vision distance prescription from a progressive one is the most straightforward conversion, as the distance correction is already explicitly stated. The SPH, CYL, and Axis values listed on the prescription form are the measurements needed for a dedicated pair of distance glasses. The purpose of this conversion is to create a lens that focuses light exclusively for objects 20 feet or further away.
For distance single-vision, the Addition (ADD) power is disregarded because it only applies to the near-vision segment of the progressive lens. For example, if a prescription for the right eye lists SPH as -2.50, CYL as -1.00, Axis as 90, and ADD as +2.00, the single-vision distance prescription will simply be -2.50, -1.00, Axis 90. The distance lens is designed to provide the best possible visual acuity at infinity. This conversion allows for a full field of sharp distance vision without the need to navigate the blended zones of a progressive lens.
Calculating the Single Vision Near Prescription
Converting a progressive prescription to a single-vision near prescription involves an algebraic calculation that incorporates the ADD power. The formula is to add the ADD value to the distance Sphere (SPH) power to find the final near-vision SPH power. For instance, if the distance SPH is -1.00 and the ADD is +2.25, the new near SPH is calculated as -1.00 + (+2.25), resulting in a final near SPH of +1.25.
This calculation is performed separately for each eye, using the distance SPH of that specific eye. The Cylinder (CYL) power and the Axis remain unchanged and are carried over directly. If the original distance prescription was -2.00 SPH, -0.50 CYL, Axis 180, and the ADD was +1.50, the near prescription would become -2.00 + (+1.50) = -0.50 SPH, with the astigmatism correction remaining at -0.50 CYL, Axis 180. The astigmatic correction does not change because astigmatism is a constant refractive error of the eye’s shape, regardless of the viewing distance.
It is important to remember that the ADD power must be combined with the spherical component of the distance prescription. The resulting single-vision lens provides maximum clarity at a typical reading distance, generally around 16 inches. This algebraic summation ensures the lens provides the total required refractive power for the eye to focus light on the retina at close range.
Specialized Uses for Converted Single Vision Lenses
The conversion of a progressive prescription is often performed to create specialized eyewear for tasks where a full progressive lens is impractical. One common application is the creation of dedicated reading glasses, which offer a much wider and distortion-free field of view for sustained close work than the reading zone of a progressive lens. This single-vision near-power lens is ideal for activities like long periods of book reading or detailed handiwork.
Another frequent use is for computer glasses, which require an intermediate focal length, typically between 20 and 40 inches. This intermediate power is often calculated by adding only a portion of the total ADD power (frequently half) to the distance SPH. For example, a +2.00 ADD power is often reduced to +1.00 for a pair of intermediate computer glasses, providing sharp focus at arm’s length.
These single-vision conversions are also valuable for safety glasses or hobby eyewear, such as glasses for shooting or playing a musical instrument. In these scenarios, a specific, constant focal length is necessary, and the constant shift in power inherent to a progressive lens can be detrimental. Creating a single-vision lens for a precise working distance ensures optimal visual comfort and performance for that dedicated activity.