Bifocal lenses contain two distinct optical powers within a single lens, offering both distance and near vision correction. The top portion corrects for viewing objects far away, while a distinct segment at the bottom provides magnification for reading and close-up tasks. The convenience and competitive pricing of online optical shops have made purchasing corrective eyewear without an in-person visit common. Successfully ordering bifocals online requires careful preparation and accurate self-measurement to ensure the lenses function as intended and fit properly. This guide outlines the specific steps necessary to translate your vision needs into a perfectly fitted pair of bifocal glasses.
Preparing Your Prescription and Data
The first step in any online glasses purchase is securing a current, valid eyeglass prescription from a licensed optometrist or ophthalmologist. Most reputable online retailers require the prescription to be less than two years old to ensure the corrective power is accurate. For bifocals, the prescription must clearly specify the Add Power, which is the supplementary magnification required for reading. This “Add” value, typically identical for both eyes, is the difference between your distance correction and your near correction.
You must accurately identify the Sphere (SPH), Cylinder (CYL), and Axis values for both the right (OD) and left (OS) eyes on your prescription form. These numbers determine the strength and orientation of the distance correction. Confirm with your eye doctor that the prescription is specifically for bifocals, as some forms may include powers for progressive lenses, which require different measurements. Entering the wrong data, especially the Add Power, will result in non-functional reading segments.
Online retailers rely entirely on the data you provide, so every detail must be confirmed before starting the order process. The prescription is the foundation of your new eyewear, detailing the precise optical correction required. Without the exact, most recent figures, the lens manufacturer cannot produce a pair of glasses that adequately corrects both distance and reading vision.
Mastering Essential Measurements
Once your prescription data is ready, you must secure two frame-dependent measurements that determine how the bifocal segment is positioned. These physical measurements are the most common source of error when ordering complex lenses online. The Pupillary Distance (PD) is the horizontal measurement that ensures the optical center of the lens aligns precisely with the center of your pupils.
PD is the distance in millimeters between the center of your two pupils, and it is crucial for proper optical centering. If your eye doctor did not provide this figure, you can measure it at home using a standard millimeter ruler and a mirror or a friend. The measurement should be taken while looking straight ahead, focusing on a distant object to keep your eyes relaxed. Most adults have a PD between 54 and 74 millimeters. Some online retailers may request the monocular PD (the distance from the nose bridge to the center of each pupil) for greater accuracy.
The second critical measurement for bifocals is the Segment Height (Seg Height), which dictates the vertical placement of the reading segment line. Seg Height is the vertical distance, measured in millimeters, from the bottom of the lens groove in the frame to where the top line of the reading segment should sit. This measurement is unique to the specific frame you have chosen and cannot be taken until you are wearing that frame.
To measure Seg Height, put on the chosen frame and look straight ahead, ensuring the frame sits naturally on your face. A friend should use a ruler to measure the distance from the bottom edge of the lens frame up to the level of your lower eyelid or the bottom of your pupil, depending on the retailer’s instructions. An incorrectly measured Seg Height will cause the reading segment to be either too high, obstructing distance vision, or too low, forcing the wearer to tilt their head to read.
Navigating the Online Purchase Process
With your prescription and physical measurements secured, you can proceed to the selection and purchasing phase. Frame selection for bifocals requires a deeper lens height than single-vision lenses to accommodate both the distance and reading segments. Frames that are too shallow may not allow for the proper placement of the bifocal line, so look for frame styles with a lens height of at least 30 millimeters.
After selecting a compatible frame, you will choose the lens material and any desired coatings. Polycarbonate is a common, impact-resistant choice. High-index plastic is often recommended for stronger prescriptions to keep the lenses thinner and lighter. Lens coatings, such as anti-reflective treatment, are recommended to reduce glare and improve visual clarity, especially when driving at night.
The final step before payment involves the precise entry of all your gathered data into the retailer’s online form. This includes the distance prescription (SPH, CYL, AXIS), the Add Power, the Pupillary Distance, and the Segment Height. Double-checking these numerical inputs against your written documents is paramount, as a single transcription error can ruin the functionality of the glasses.
Once the order is placed, some online companies may contact you to verify the prescription details before manufacturing begins. You should carefully review the retailer’s return and adjustment policy, especially concerning errors in self-reported measurements like Seg Height. Knowing the policy provides a safeguard in case the bifocal line placement is incorrect upon arrival, ensuring you can request an adjustment or a refund if the glasses are unusable.