Optical glossary
Plain-English definitions for the terms on a prescription, a lens quote, and across this site.
- terms
- 46terms
- themes
- 8themes
- diagrams
- 33diagrams
01 · 6 terms
Reading a prescription
The numbers your optometrist writes down, column by column: what each one corrects, and how they combine into one lens.
Illustrative · the shape of the principle, not measured data
- DiopterThe unit of lens power. One diopter focuses parallel light at one meter.
- Sphere (SPH)The main power of your prescription, in diopters. Minus corrects nearsightedness, plus corrects farsightedness; further from zero means stronger.
- Cylinder (CYL)The correction for astigmatism, where the eye's optics focus light along two different lines instead of one point. Always paired with an axis.
- AxisThe orientation of your astigmatism correction, from 1 to 180 degrees. A cylinder value without an axis is meaningless.
- Addition (ADD)The extra reading power added to the lower part of a progressive or bifocal. Typically starts near +1.00 in the early forties and usually settles near +2.50 (up to +3.00 for close working distances).
- PrismA deliberate offset that shifts an image's position, prescribed for eye-alignment problems. Unwanted prism from bad centration causes strain.
02 · 4 terms
How eyes focus
The four conditions behind almost every pair of glasses, and what each one asks of a lens.
Illustrative · the shape of the principle, not measured data
- MyopiaNearsightedness: distance is blurred because light focuses in front of the retina. Corrected with minus lenses.
- HyperopiaFarsightedness: light would focus behind the retina, making near work effortful. Corrected with plus lenses.
- AstigmatismAn eye whose optics are curved unequally in different directions, so light focuses along two lines rather than one point. Extremely common and fully correctable with a cylinder.
- PresbyopiaThe age-related stiffening of the eye's internal lens that makes close focus difficult, typically from the early forties. Not a disease — the reason reading additions and progressives exist.
03 · 5 terms
Kinds of lens
One power, two powers, or a continuous change between them — and the trade each design makes to get there.
Illustrative · the shape of the principle, not measured data
- Single vision lensOne power across the whole lens, for one viewing distance.
- BifocalA lens with two distinct zones separated by a visible line, and no intermediate correction between them.
- Progressive lensA lens whose power changes gradually from distance at the top to reading at the bottom, with no visible line.
- CorridorThe channel of clear vision running down the center of a progressive, connecting the distance and reading zones.
- Peripheral distortionThe soft, swimming zones beside a progressive's corridor — a geometric consequence of continuous power change, present in every progressive ever made. Better designs manage it; none eliminate it.
04 · 9 terms
Measuring and fitting
The measurements that place a lens in front of your eye. A perfect prescription in the wrong place makes a poor pair of glasses.
Illustrative · the shape of the principle, not measured data
- Pupillary distance (PD)The distance between your pupil centers in millimeters. It positions each lens's optical center in front of each eye; errors cause strain even with a correct prescription.
- Monocular PDYour PD measured separately for each eye, from the nose bridge. More accurate than a single figure, and required for individualized lenses.
- Optical centerThe point of a lens where light passes without deviation. Should sit directly in front of your pupil.
- Fitting heightThe vertical distance from your pupil center to the bottom of the lens in your chosen frame. The most common cause of progressive remakes when wrong.
- Frame measurementsThe code inside the temple arm (e.g. 51□19-145): lens width, bridge width, and temple length in millimeters. More than frame trivia — lens width drives edge thickness at stronger prescriptions, and together with the bridge it determines how far the lenses must shift to center on your pupils.
- Vertex distanceThe gap between the back of the lens and your eye. At strong powers, a change in this distance changes the effective prescription.
- Pantoscopic tiltThe forward tilt of a frame relative to the face. Individualized designs account for it; standard designs assume an average.
- Wrap angleHow much a frame curves around the face. High-wrap sports frames need designs compensated for it.
- Base curveThe curvature of a lens's front surface. Must suit both the prescription and the frame's wrap.
05 · 5 terms
Lens materials
What the index number buys, what it costs in optical terms, and the materials behind each step.
Estimated edge-thickness reduction
- 1.50
Standard referenceStandard 1.50 material
- 1.56
Up to 10% thinnerthan 1.50 at −6.00, indicative
- 1.60
Up to 20% thinnerthan 1.50 at −6.00, indicative
- 1.67
Up to 30% thinnerthan 1.50 at −6.00, indicative
- 1.74
Up to 40% thinnerthan 1.50 at −6.00, indicative
Dashed outline: the same power in 1.50Schematic shapes — not to scale, not a cross-section of a finished lens
- Index 1.50: Standard reference
- Index 1.56: Up to 10% thinner
- Index 1.60: Up to 20% thinner
- Index 1.67: Up to 30% thinner
- Index 1.74: Up to 40% thinner
Relative comparison · indicative percentages against 1.50, not measurements
- Refractive indexHow strongly a material bends light. Higher index means less material for the same power — a thinner lens.
- High indexIn the CLIFF range, materials of 1.67 and above, used to keep strong prescriptions thin. Buys thinness, not clarity.
- MR-8The tough mid-index material of CLIFF's 1.60 lenses — impact-resistant enough for drilled rimless mounting and the range's breakage warranty.
- Abbe numberA measure of how much a material disperses light into colors. It falls as index rises; lower Abbe can mean faint color fringing at the edges of strong lenses.
- DispersionThe splitting of white light into colors as it passes through a material. The optical price of high index.
06 · 10 terms
Coatings and filters
The layers on a lens, and the light each one stops, softens or lets through.
Illustrative · the shape of the principle, not measured data
- Hard coatingA hardened protective layer that resists scratching. The foundation every other coating is built on.
- Anti-reflective (AR) coatingMicroscopic layers tuned so surface reflections cancel. More light through the lens, no bright rectangles over your eyes, no headlight starbursts.
- Residual reflectionThe faint color an AR coating still reflects — green, gold or blue depending on the layer design. A signature, not a flaw.
- Hydrophobic / easy-clean layerA slick top layer that makes water bead and smudges wipe away rather than smear.
- UV400Full ultraviolet protection to 400 nanometers. Invisible: darkness and UV protection are unrelated properties.
- Blue light filteringAttenuation of the blue end of the visible spectrum, associated with reduced eye fatigue on long screen days. The evidence supports comfort, not medical protection.
- TintA fixed color in or on a lens. Cosmetic or comfort — a tint alone is not UV protection.
- PhotochromicA lens that darkens in ultraviolet light and clears indoors. Most photochromic lenses react poorly behind windshields, which block most UV.
- Spin coatingApplying the photochromic layer onto the lens surface at even thickness, so color depth stays uniform at any prescription. In-mass photochromics carry the chemistry within the material instead — a difference that shows mainly at stronger prescriptions, where thickness varies more across the lens.
- PolarizedA lens with a filter that blocks the horizontally-reflected light that reads as glare off water, roads and glass. Permanently tinted.
07 · 4 terms
Making and checking a lens
How a lens surface is cut for one order, how it is marked, and how its strength is proven.
Illustrative · the shape of the principle, not measured data
- Semi-finished blankA lens with a factory-molded front surface, finished by surfacing the back to the prescription. Every CLIFF Rx lens starts as one: freeform surfacing is how its back is finished.
- Freeform surfacingManufacturing in which a lens surface is cut point by point by a computer-guided tool from a map calculated for one order, rather than molded. On made-to-order CLIFF lenses it is the back surface — the surfacing and the design both — on every made-to-order tier. Versatile, the stock lens, is molded, not freeform.
- EngravingPermanent laser markings applied at manufacture, readable under angled light. Every stock lens and every progressive lens carries the CLIFF mark — the CLIFF icon, or the letters CLF.
- Drop-ball testAn impact-resistance test: a lens must survive a steel ball dropped onto it from a set height.
08 · 3 terms
From order to wear
The professional work between a prescription and glasses that feel right — and what happens when they don't.
Illustrative · the shape of the principle, not measured data
- DispensingThe professional work between prescription and finished glasses: frame selection, measurement, fitting, and adjustment. Where most of a lens's real-world performance is decided.
- RemakeA lens made again due to error or non-adaptation. The most common causes: fitting height and frame changes after measurement.
- Non-adaptationThe inability to adjust to a progressive design. Rare, usually fixable with a different design — and covered under the CLIFF Promise for 30 days.
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