Walk into an optical shop and you’ll hear words you’ve never used in your life. CYL. PD. Acetate. Hi-index. This eyewear glossary is the plain-language version — every term you’ll actually encounter when buying glasses, written by a licensed optician.
The piece across the nose is the bridge. The arms that go back to your ears are the temples. Where the temple meets the front of the frame is the end piece, and the small joint between them is the hinge.
Each lens sits inside a rim, which on metal frames is sometimes called the eye wire. Some frames — most often aviators and certain metal styles — add a horizontal bar across the top of the bridge. That’s a brow bar, also called a top bar or double bridge. On most metal frames you’ll find nose pads, the small plastic or silicone pieces that touch your nose, held in place by pad arms. Acetate frames usually skip nose pads in favor of a sculpted bridge built into the frame itself.
Pick up any pair of glasses and look on the inside of the temple. You’ll see something like 52-18-145. Those numbers are in millimeters: eye size (lens width), bridge size (distance between lenses), and temple length (the full arm length).
Two more numbers matter for fit but aren’t always printed: lens height, which determines whether progressives or bifocals will work, and total frame width, which is the full edge-to-edge measurement and the single most useful number when comparing whether a frame will suit your face.
The sphere (SPH) corrects near or farsightedness. A negative number means nearsighted, a positive number means farsighted. Your cylinder (CYL) and axis (AX) appear together when you have astigmatism — cylinder is the strength, axis is the orientation in degrees.
If you wear progressives or bifocals, you’ll also see an add power (ADD), which is how much extra magnification you need for reading. OD means right eye, OS means left eye, OU means both. Your pupillary distance (PD) is the distance in millimeters between your pupils — without it, lenses can’t be centered correctly.
“Most people don’t need to know any of this. But when something goes wrong, the people who do know save themselves a lot of money.”
Single vision lenses have one prescription across the whole lens — most people under 40 wear these. Progressive lenses transition smoothly from your distance prescription at the top to your reading prescription at the bottom, with no visible line.
Bifocals do the same job as progressives but with a visible dividing line between the two zones. Reading lenses are single vision lenses set to your reading prescription only — the kind you find at the pharmacy.
Polycarbonate is the most impact-resistant common lens material — the standard for kids’ eyewear, sports, safety glasses, and rimless frames where the lens is screwed in. It’s lightweight and shatter-resistant, with a refractive index of 1.586. Trivex is a separate material — a urethane-based pre-polymer originally developed for the military. It’s the lightest lens material available, with cleaner optics than polycarbonate and impact resistance that meets the same ANSI Z87.1 safety standard.
Hi-index 1.67 is the workhorse premium lens material, well-suited to prescriptions in the -4.00 to -7.00 range. The lenses are noticeably thinner than standard plastic, your eyes don’t appear shrunken behind the lens, and the frame sits lighter on your face. Hi-index 1.74 is one of the thinnest lens materials commonly available, generally reserved for prescriptions stronger than ±7.00 to ±8.00, or for thin metal and rimless frames where every millimeter of edge thickness shows.
Anti-glare (also called AR coating or anti-reflective) is the single most useful treatment — it eliminates the reflections you see in photos and reduces eye strain at night. Blue light filter reduces a portion of high-energy visible light from screens.
Photochromic lenses darken outdoors when exposed to UV light and clear indoors. We use the Transitions brand. One important detail: standard Transitions lenses don’t darken behind a car windshield because windshields block most UV light. Specific variants — Transitions XTRActive in particular — are engineered to react to visible light too, which lets them activate moderately in the car. Polarized lenses cut horizontal glare from water, snow, and roads. Polarized lenses are tinted by nature, so they’re sunglass-only. Mirror coatings are cosmetic on the front surface; they don’t change how you see, only how others see your lenses.
Acetate (sometimes called zyl) is a plant-based plastic made from cotton and wood pulp, layered in sheets to create depth and color. The good stuff comes from Italy or Japan. Injection plastic is mass-produced low-grade plastic — generally avoid if you can.
Monel is a copper-nickel alloy used in mid-tier metal frames. Stainless steel is more rigid and more corrosion-resistant than monel. Pure titanium is lightweight, hypoallergenic, and corrosion-proof — premium territory. Note that not every frame marketed as “titanium” is pure; many are titanium alloys. Beta-titanium is a flexible variant used in temples. Inside many acetate frames is a thin metal wirecore, allowing the temple to be adjusted without snapping.
PD — pupillary distance. OD / OS / OU — right eye, left eye, both. SPH / CYL / AX — sphere, cylinder, axis. ADD — additional reading power. DV / NV — distance vision, near vision.
Base curve is the curvature of the front of the lens — flatter for high prescriptions, more curved for sunglasses. Vertex distance is the distance from your eye to the back surface of the lens, and it matters when prescriptions are very strong.
Unfortunately, we don't have any direct partnerships with vision insurance providers, but we can assist you in getting a reimbursement for your purchase after you have paid in full at the time of purchase. In order to be reimbursed, you will first need to fill out a form from your insurance provider. After you fill out their reimbursement form, you will need to provide an itemized receipt, which we can provide you with once you have completed your purchase. We've provided some links below of popular vision insurance providers to help you get started.
Please contact your vision insurance for more information on how to file for a reimbursement.
If you have any question or you need an itemized receipt, please email us at info@nerdyframes.com
We accept both FSA and HSA payments, as long as they are in the form of a credit or debit card. After you have had completed a purchase with your FSA/HSA, you can then file for a reimbursement with your vision insurance provider.
