A bifocal prescription contains everything a standard eyeglass prescription does, plus one extra value: the ADD power, which tells the lab how much magnification to build into the lower reading portion of your lenses. Once you know where to find each value and what it means, the whole prescription becomes straightforward to decode.
OD and OS: Which Eye Is Which
Every prescription is split into two rows, one for each eye. OD (oculus dexter) is your right eye, and OS (oculus sinister) is your left eye. These Latin abbreviations are standard across virtually all prescriptions, whether printed or handwritten. Some offices use RE and LE instead, but OD and OS remain the most common. If you see OU, that means both eyes together.
Sphere, Cylinder, and Axis
These three columns describe your distance vision correction, measured in units called diopters.
Sphere (SPH) is the main correction your eyes need. A minus sign (like -2.50) means you’re nearsighted. A plus sign (like +1.75) means you’re farsighted. The higher the number in either direction, the stronger the lens.
Cylinder (CYL) corrects astigmatism, which happens when the front of your eye is shaped more like an oval than a perfect sphere. Not everyone has a value here. If your CYL column is blank or reads “SPH,” you don’t have astigmatism in that eye. When a number is present, it works alongside the sphere value to account for the irregular curvature.
Axis is a number between 0 and 180 that only appears when there’s a cylinder value. It pinpoints the angle of your astigmatism on the cornea, telling the lab exactly how to orient the correction in the lens. Think of it as a compass heading for the oval shape of your eye.
The ADD Value: What Makes It a Bifocal
The ADD (sometimes written as “Add Power” or “NV-ADD”) is the single value that separates a bifocal prescription from a regular one. It represents the extra magnifying power layered into the bottom segment of your lens so you can read up close. This number is always positive, even if no plus sign is printed, and it’s the same for both eyes in nearly all cases.
ADD values typically fall between +0.50 and +3.50, increasing in quarter-diopter steps (+1.00, +1.25, +1.50, and so on). A lower ADD means you need only a small boost for reading. A higher ADD means your eyes’ built-in focusing ability has declined more significantly.
Your ADD power is not added to your sphere by you. The optical lab combines the two values when grinding the reading portion of the lens. So if your right eye has a sphere of -3.00 and an ADD of +2.00, the distance zone of that lens is -3.00 and the reading zone is -1.00. You don’t need to do that math yourself, but understanding it helps you see why the bottom of your bifocals makes close objects clear while the top keeps distant objects sharp.
How ADD Power Changes With Age
Bifocal prescriptions exist because of presbyopia, the gradual loss of up-close focusing that starts in your early to mid-40s. Your ADD power will almost certainly increase over time as the lens inside your eye continues to stiffen.
People in their 40s commonly start with an ADD around +1.00 to +1.25. By the 50s, that typically climbs to roughly +2.00 to +2.50. In the 60s, most people land between +2.50 and +3.00, and the number usually plateaus somewhere in that range. If your ADD jumped from your last prescription, that’s normal progression rather than a sign of a new problem.
Pupillary Distance on a Bifocal Prescription
Pupillary distance (PD) measures how far apart your pupils are, in millimeters. Every pair of glasses needs this measurement so the optical center of each lens sits directly in front of your pupil. For bifocals, there’s an added layer: you may need both a distance PD and a near PD.
When you look at something close, your eyes naturally converge inward, making your effective pupillary distance a bit smaller. A quick rule of thumb: your near PD is about 3 mm less than your distance PD. If your distance PD is 64 mm, your near PD is roughly 61 mm. Some prescriptions list a single PD per eye (called dual or monocular PD), in which case you subtract 1.5 mm from each side for near work. Getting these numbers right matters because a misaligned reading segment can cause eye strain or blurry close-up vision.
Segment Height and Segment Width
Two measurements that may appear on your prescription or your order form (but not always) deal with the physical placement of the reading area in your lens.
Segment height (Seg Height) tells the lab how high to position the reading portion relative to the bottom of your frame. It’s measured from the lowest edge of the lens up to where the reading segment begins, and it’s based on where your pupil sits when wearing the frame you’ve chosen. If the seg height is too high, the reading zone creeps into your line of sight when looking straight ahead, which can make driving uncomfortable. Too low, and you have to tilt your head way back to find the reading area.
Segment width describes how wide the reading window is. Standard flat-top bifocals (sometimes labeled FT28) have a 28 mm wide reading segment. If you need more reading area, perhaps because you spend a lot of time at a computer or reading sheet music, an FT35 lens widens that segment to 35 mm. Your eye care provider or optician will recommend the appropriate width based on your visual needs.
Putting It All Together
Here’s what a complete bifocal prescription might look like on paper:
- OD: SPH -2.25, CYL -0.75, Axis 090, ADD +2.00
- OS: SPH -1.50, CYL -0.50, Axis 180, ADD +2.00
- PD: 63 mm
Reading this line by line: the right eye needs a -2.25 diopter correction for distance, with -0.75 diopters of astigmatism correction oriented at 90 degrees. The left eye needs a slightly weaker distance correction with less astigmatism at a different angle. Both eyes get an additional +2.00 diopters of magnification built into the reading segment. The lenses are centered for a pupillary distance of 63 mm.
If any field on your prescription is blank, it simply means you don’t need correction in that category for that eye. A blank CYL and Axis means no astigmatism. A blank ADD means no bifocal, so if you expected bifocals and don’t see an ADD value, it’s worth double-checking with your provider.
Bifocal vs. Progressive Prescriptions
The prescription itself is identical for bifocals and progressives. Both use the same ADD value. The difference is in how the lens is manufactured. Bifocals have a visible line separating the distance and reading zones, with an abrupt jump between the two. Progressives blend the transition gradually with no visible line, adding an intermediate zone for tasks like computer work. Your prescription doesn’t change based on which lens style you choose. The ADD power, sphere, cylinder, and axis stay the same either way.