Glossary of optical terms

Short definitions of the terms found on an eye prescription, on a lens order and on a frame. Where a term has a formula, the definition links to the free calculator that uses it.

General optics for opticians, students and shop staff, by the team behind Asaan Optics. It is not advice about anyone's eyes; see the medical and calculation disclaimer. Updated 10 October 2026.

Prescription terms

Sphere (SPH)
The main lens power, in dioptres, that corrects short sight or long sight. A minus value corrects myopia and a plus value corrects hyperopia. It acts equally in every direction of the lens.
Cylinder (CYL)
The extra lens power, in dioptres, that corrects astigmatism. It acts in one direction only, which the axis sets. It can be written in plus or minus form; both describe the same lens. Convert between them with the plus to minus cylinder converter.
Axis
The direction of the cylinder, in degrees from 1 to 180. 90 is vertical and 180 is horizontal. An axis is written only when there is a cylinder.
Addition (ADD)
Extra plus power added to the distance prescription for near work. It is used for reading glasses, bifocals and progressive lenses, and is usually the same for both eyes. The distance to reading converter applies it.
Pupillary distance (PD)
The distance between the centres of the two pupils, in millimetres. It is written as one figure for both eyes (binocular PD) or one figure for each eye, measured from the middle of the nose (monocular PD). Lenses are centred with it. Near PD is a few millimetres smaller than distance PD.
Prism
Lens power that shifts the image so the two eyes work together. It is measured in prism dioptres (Δ) and written with a base direction. Two prisms can be combined into one with the compounding prism calculator.
Base (of a prism)
The thickest edge of a prism, given as in, out, up or down. Light is bent towards the base, so the image appears to move towards the opposite edge, the apex.
Dioptre (D)
The unit of lens power. It is one divided by the focal length in metres: a +2.00 D lens focuses distant light at 0.5 m.
Plano (PL)
Zero power. It is written when an eye needs no sphere correction.
OD, OS and OU
Latin abbreviations. OD (oculus dexter) is the right eye, OS (oculus sinister) is the left eye and OU (oculus uterque) is both eyes. Many prescriptions use R and L instead.
DV and NV
Distance vision and near vision. They label which part of a prescription is for far and which is for close work.
Visual acuity (VA)
How small a detail the eye can make out, written as a fraction such as 6/6 (metres) or 20/20 (feet). 6/12 means the person reads at 6 m what a person with standard vision reads at 12 m.
Spherical equivalent
One sphere power that stands in for a sphere and cylinder prescription: the sphere plus half the cylinder. See the spherical equivalent calculator.
Transposition
Rewriting a prescription from plus-cylinder to minus-cylinder form or back. Add the cylinder to the sphere, change the sign of the cylinder, and turn the axis by 90 degrees. The transposition calculator does it.
Vertex distance (BVD)
The distance from the back surface of a spectacle lens to the front of the eye, usually about 12 to 14 mm. It changes the power the eye receives, which matters for stronger prescriptions and when converting spectacles to contact lenses. See the vertex distance calculator.

Conditions a prescription corrects

Myopia (short sight)
Distant objects look blurred because the eye focuses light in front of the retina. It is corrected with minus sphere power.
Hyperopia (long sight)
The eye focuses light behind the retina, so near work, and sometimes distance, needs extra effort or looks blurred. It is corrected with plus sphere power.
Astigmatism
The eye has different focusing power in different directions, usually because the cornea is not evenly curved. It is corrected with cylinder power at a set axis.
Presbyopia
The gradual loss of the eye's ability to focus up close with age. It is corrected with an addition, in reading glasses, bifocals or progressive lenses.
Anisometropia
A marked difference in prescription between the two eyes.

Lens terms

Refractive index
How strongly a lens material bends light. Standard plastic (CR-39) is about 1.50. Higher-index materials such as 1.60, 1.67 and 1.74 give a thinner lens for the same power.
Abbe value
A measure of how much a material splits white light into colours. A higher Abbe value means less colour fringing at the edge of the lens.
Base curve
The curve of the front surface of a spectacle lens, in dioptres. For a contact lens it is the radius of the back surface, in millimetres. See the surface curve calculator.
Centre and edge thickness
The thickness of a lens at its middle and at its rim. Minus lenses are thickest at the edge and plus lenses in the centre. Estimate both with the lens thickness calculator.
Optical centre
The point of a lens through which light passes without being bent. It is normally placed in front of the pupil.
Decentration
Moving the optical centre away from the middle of the lens shape so that it sits in front of the pupil.
Prentice's rule
Looking through a lens away from its optical centre produces prism: prism (Δ) equals lens power (D) times the distance from the centre in centimetres. See the induced prism calculator.
Minimum blank size
The smallest uncut lens that will fill a frame once it has been decentred: the effective diameter of the frame plus twice the decentration, plus a small allowance. See the minimum blank size calculator.
Pantoscopic tilt
The tilt of a fitted lens, with its lower edge closer to the cheeks than its upper edge. Tilt changes the effective power of stronger lenses; the lens tilt and wrap calculator shows by how much.
Wrap angle (face form)
How far the front of a frame curves around the face. Like tilt, a large wrap changes the effective power of the lens.
Fitting height and segment height
The vertical distance from the lowest point of the lens in the frame up to the pupil centre (for progressive lenses) or to the top of the reading segment (for bifocals).
Toric lens
A lens with two different powers at right angles to each other. It is how spectacle and contact lenses correct astigmatism.

Lens types and coatings

Single vision
A lens with one prescription across its whole surface, for distance, for near, or for an in-between working distance.
Bifocal
A lens with two powers: distance in the main part and near in a visible segment low in the lens.
Progressive lens (varifocal, PAL)
A lens whose power changes smoothly from distance at the top to near at the bottom, with no visible line.
Corridor
The narrow channel in a progressive lens along which the power changes from distance to near.
Photochromic
A lens that darkens in ultraviolet light outdoors and clears again indoors.
Polarised
A tinted lens with a filter that cuts glare reflected from flat surfaces such as water and roads.
Anti-reflective (AR) coating
A thin coating that cuts reflections from the lens surfaces, so more light passes through and the lens looks clearer.
Hard coat
A scratch-resistant coating on plastic lenses.
UV400
A lens that blocks ultraviolet light up to a wavelength of 400 nanometres, which covers UVA and UVB.

Frame measurements

Boxing system
The standard way to measure a frame: each lens shape is enclosed in a rectangle, and the frame sizes are taken from those rectangles.
Eye size (A)
The horizontal width of the lens rectangle, in millimetres. It is the first number printed on a frame, as in 52-18-140.
B measurement
The vertical height of the lens rectangle, in millimetres.
Bridge (DBL)
The distance between the two lenses, in millimetres. It is the second number printed on a frame.
Temple length
The length of the side arm, in millimetres. It is the third number printed on a frame.
Effective diameter (ED)
Twice the longest distance from the centre of the lens shape to its edge. It is used to work out the minimum blank size.
Frame PD
The distance between the centres of the two lens shapes: eye size plus bridge. Comparing it with the wearer's PD gives the decentration.

Contact lens terms

Base curve (BC)
The radius of the back surface of a contact lens, in millimetres. It is chosen to suit the curve of the cornea.
Diameter (DIA)
The width of a contact lens from edge to edge, in millimetres.
Keratometry (K readings)
Measurements of the curve of the front of the cornea in its two main directions. They are used when fitting contact lenses.

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