Looking at an eyeglass prescription or a new pair of lenses can be confusing, especially when trying to determine if they are single vision or progressive. Understanding the difference is important for placing orders, managing costs, and ensuring your vision is corrected precisely. The distinction can be found through specific notations on your prescription slip or by physically inspecting the lenses themselves.
Defining Single Vision and Progressive Lenses
Single Vision (SV) lenses provide a single, uniform prescription power across the entire lens surface. This power corrects vision for one specific distance, such as far distance for driving or near distance for reading. If you are only nearsighted or farsighted, or need correction for a single task, you likely use SV lenses.
Progressive Addition Lenses (PALs), often called progressives, are multifocal lenses that correct vision for multiple distances within a single lens. Unlike traditional bifocals, progressives offer a seamless, graduated change in power from the top to the bottom without visible lines. The top section is for distance viewing, the middle corridor provides intermediate vision (like for a computer screen), and the bottom area contains the full reading power for near tasks.
Locating the “ADD” Power on Your Prescription
The most definitive way to identify a multifocal lens prescription is to check for the “ADD” power. “ADD” stands for “Addition,” and its presence indicates the extra magnifying power needed for near vision. This power is necessary to manage presbyopia, the age-related loss of the eye’s ability to focus on close objects, which typically affects people after age 40.
On a standard prescription form, you will find columns for Sphere, Cylinder, and Axis, followed by a separate column labeled “ADD” or “NV-ADD” (Near Vision Add). The numbers in the ADD column are always positive, typically ranging from +0.75 to +3.00 diopters. This value is added to the distance prescription to achieve the reading power. If the ADD column contains a numerical value, your prescription is for a multifocal lens (progressive, bifocal, or trifocal).
If the ADD column is blank, contains “PL” (Plano), or “0.00,” the prescription is for Single Vision lenses only. The absence of a number means the distance power is the only correction applied to the lens.
How to Physically Inspect Your Lenses
If you have the glasses but not the original prescription slip, you can perform two checks to determine the lens type. The first is a visual test: look through the top of the lens at a distant object, then slowly lower your gaze to look through the bottom at close-up text. If the lens is single vision, the distant object and the close-up text will be clear in one section but out of focus in the other.
If the lens is a progressive, the focus will smoothly transition from clear distance vision at the top to clear near vision at the bottom. This seamless change in power is the hallmark of a progressive lens design. You may also notice a slight “wavy” distortion or blurriness when looking through the far edges, which is a byproduct of blending the different optical zones.
The second method involves looking for nearly invisible micro-engravings on the lens surface. Professional progressive lenses have permanent, laser-etched markings used for verification and identification, though they are often difficult to see. These etchings typically include the manufacturer’s logo and the full “ADD” power, located near the temporal and nasal edges of the lens. By holding the lens up to a strong light source against a dark background and tilting it slightly, you can often catch the reflection of these faint engravings, which confirm the lens type and reveal the exact addition power.
Measurements Unique to Progressive Lenses
Progressive lenses require specific measurements that are not necessary for single vision lenses because the different optical zones must be precisely aligned with the wearer’s eyes. One important unique measurement is the Fitting Height, sometimes called the Segment Height. This is the vertical measurement, given in millimeters, from the center of the pupil down to the deepest point of the lens within the frame.
The Fitting Height dictates where the progressive corridor begins, ensuring the distance, intermediate, and near viewing zones are positioned correctly for the wearer’s natural line of sight. Single vision lenses do not require a fitting height.
Furthermore, while single vision prescriptions often use a single PD measurement, progressive lenses usually require Monocular PDs. Monocular PDs measure the distance from the center of the nose to the center of each pupil separately. This separate measurement ensures the progressive zones are centered with maximum precision, which is required for optimal performance.