Prescription glasses are corrective lenses custom-made to compensate for the specific way your eyes bend light. Every pair is built around a unique prescription, a set of measurements that tells the lens manufacturer exactly how to shape each lens so that light focuses precisely on your retina, the light-sensitive tissue at the back of your eye. If you’ve recently been told you need glasses, or you’re just curious about how they work, here’s what’s actually going on.
How Prescription Lenses Correct Your Vision
Your eye works like a camera. Light enters through the cornea and lens at the front, which bend the light so it converges into a sharp point on the retina. When the shape of your eyeball or cornea is slightly off, that focal point lands in the wrong place, and your vision blurs. Prescription lenses fix this by redirecting light before it enters your eye.
There are a few common ways your eye’s optics can be off:
- Nearsightedness (myopia): Your eyeball is slightly too long, so light focuses in front of the retina instead of on it. Distant objects look blurry. A concave lens (thinner in the center, thicker at the edges) spreads the light outward just enough to push the focal point back onto the retina.
- Farsightedness (hyperopia): Your eyeball is too short, so light focuses behind the retina. Close-up tasks like reading become difficult. A convex lens (thicker in the center) converges the light more tightly so it reaches focus sooner.
- Astigmatism: Your cornea is curved unevenly, like a football instead of a basketball. This creates two different focal points, which means both near and far objects can look stretched or blurred. A cylindrical lens corrects one curve without affecting the other.
These aren’t rare conditions. Most people who wear glasses have some combination of myopia or hyperopia with a degree of astigmatism mixed in.
Reading Your Prescription
An eyeglasses prescription looks like a small table of abbreviations and numbers. Each value tells the lab something specific about how to grind your lenses. According to the American Academy of Ophthalmology, the key terms break down like this:
- SPH (Sphere): The main corrective power of the lens, measured in diopters. A minus sign means you’re nearsighted; a plus sign means you’re farsighted. The higher the number, the stronger the correction.
- CYL (Cylinder): The amount of astigmatism correction needed. If this box is empty, your cornea is evenly curved and you don’t have astigmatism.
- AXIS: A number between 1 and 180 that indicates the angle of your astigmatism on the cornea. It tells the lab exactly how to orient the cylindrical correction in the lens.
- ADD: Extra magnifying power added to the bottom portion of the lens for reading. This value typically appears on prescriptions for people over 40 who need help with close-up vision, a condition called presbyopia.
Your prescription will have separate rows for your right eye (OD) and left eye (OS), since each eye usually needs a slightly different correction. Prescriptions for glasses expire, typically after one to two years, because your eyes can change over time.
Lens Materials and How They Differ
The material your lenses are made from affects their weight, thickness, and durability. Each material bends light differently, which determines how much physical material is needed to achieve your correction.
CR-39 is the classic plastic lens material. It provides good optical clarity and works well for mild prescriptions where thickness isn’t a concern. It’s reasonably durable for everyday use but not impact-resistant, so it can crack if hit hard. It’s not recommended for children, sports, or safety eyewear.
Polycarbonate is significantly lighter and thinner than CR-39. Originally developed for safety equipment, it’s highly impact-resistant and virtually shatterproof, which makes it the standard choice for kids’ glasses, sports eyewear, and anyone who needs extra durability. If you wear your glasses all day, the lighter weight also makes a noticeable comfort difference.
High-index lenses bend light more efficiently than either CR-39 or polycarbonate. Because less material is needed to achieve the same correction, these lenses come out noticeably thinner and lighter, especially for moderate to strong prescriptions. If your SPH number is high (say, -6.00 or more), high-index lenses can be the difference between thick, heavy lenses and something that looks and feels more proportional to your frame. They’re less impact-resistant than polycarbonate but hold up fine for everyday adult use.
Single Vision, Bifocals, and Progressives
If your prescription only corrects for one distance (near or far), you’ll get single-vision lenses. One uniform correction across the entire lens. Most people under 40 wear single-vision glasses.
As you get older and your eyes lose the ability to shift focus between distances, you may need correction for more than one range. That’s where multifocal lenses come in. Bifocals have two zones divided by a visible line: the top corrects distance vision for things like driving or watching TV, while the bottom provides close-up power for reading or phone use. Trifocals add a third zone in the middle for intermediate tasks like computer work, separated by two visible lines.
Progressive lenses do the same job as trifocals but without any visible lines. They blend smoothly from distance correction at the top through intermediate in the middle to reading power at the bottom. The trade-off is a short adjustment period. Your peripheral vision may feel slightly distorted for the first week or two as your brain learns to look through the right part of the lens for each distance.
Common Coatings and Add-Ons
Most prescription glasses today come with optional coatings that improve comfort or durability.
Anti-reflective coating reduces the amount of light that bounces off the lens surface instead of passing through it. Uncoated glass reflects roughly 4% of incoming light per surface. A good anti-reflective coating drops that below 1%, which means more light reaches your eyes, glare from headlights and screens is reduced, and your lenses look clearer instead of showing distracting reflections. It also makes your eyes more visible to others, which matters for video calls and photos.
Scratch-resistant coatings add a thin, hard layer to protect against everyday wear. No coating makes lenses completely scratch-proof, but it significantly extends their life. Most polycarbonate and high-index lenses come with scratch resistance included because the base material is softer than CR-39.
Photochromic lenses (often called by the brand name Transitions) darken automatically in sunlight and fade back to clear indoors. They work through a reversible chemical reaction triggered by UV light. One practical thing to know: they respond to temperature. In cold weather, the lenses take considerably longer to fade back to clear. Testing has shown that the time to return to 80% transparency at cold temperatures (around 40°F) is roughly six times longer than at room temperature. They also don’t darken well inside a car, since windshields block most UV light.
Do Blue Light Glasses Actually Work?
Blue light filtering lenses have been heavily marketed for reducing eye strain from screens. The evidence doesn’t support the claims. A large systematic review published in 2023 found that blue light filtering lenses likely provide no meaningful benefit for eye strain symptoms during computer use, and no clinically significant improvement in visual performance or sleep quality compared to regular lenses. Digital eye strain is real, but it’s driven by prolonged focusing at a fixed distance and reduced blinking, not by blue light specifically. Taking breaks and adjusting your screen distance does more than a blue light coating.
That said, if your prescription glasses already come with a blue light filter bundled into another coating, it won’t hurt anything. It’s just not worth paying extra for as a standalone feature.
Getting the Right Fit
A prescription is only half the equation. How the glasses sit on your face determines whether the lenses actually work as intended. The optical center of each lens needs to align with your pupils. If your frames are too wide, too narrow, or sitting at the wrong height, you may experience blurred edges, headaches, or a sense that your vision “isn’t quite right” even though the prescription is accurate.
When you pick up new glasses, the optician will measure your pupillary distance (the space between your pupils, in millimeters) and adjust the frame so the lenses center correctly. If you order glasses online, you’ll need this measurement yourself. You can ask for it during your eye exam, or measure it at home using a ruler and a mirror, though the in-office measurement is more precise. Frame adjustments, like bending the temple arms or adjusting nose pads, are typically free at optical shops even if you didn’t buy your glasses there.