Understanding Your Vision Prescription

Each eye gets values for sphere, cylinder and axis, sometimes with additions for reading or prism. Understanding these makes it possible to compare quotes and to know whether an online retailer can supply what you need.

Sphere, cylinder and axis

Sphere, abbreviated SPH, is the main correction measured in dioptres. A minus value indicates myopia, or short-sightedness, where distant objects blur. A plus value indicates hyperopia, or long-sightedness. Larger absolute numbers mean stronger correction.

Cylinder, or CYL, corrects astigmatism, where the cornea or lens is shaped more like a rugby ball than a football, causing blur at all distances. Not everyone has a cylinder value; many people have a small one.

Axis accompanies cylinder and is a number from 1 to 180 describing the orientation of the astigmatism correction in degrees. It is meaningless without a cylinder value and must be precise, since a lens with the correct power at the wrong axis will not work.

Add, prism and pupillary distance

ADD is the additional plus power for reading, appearing in bifocal, trifocal and progressive prescriptions. It is typically the same for both eyes and increases gradually with age as presbyopia progresses.

Prism corrects eye alignment problems and is written with a direction — base in, base out, base up or base down. It is relatively uncommon and requires accurate measurement.

Pupillary distance, or PD, is the distance between pupil centres in millimetres. It is a measurement rather than part of the prescription, but it is required to make glasses correctly, and it is the item online buyers most often lack. Optical practices are not always obliged to provide it, though many will.

Presbyopia and why reading glasses appear in your forties

The lens inside the eye stiffens progressively with age, reducing its ability to change focus for near work. This is presbyopia, it is universal, and it typically becomes noticeable in the early to mid forties.

Options include reading glasses for near work only, bifocals with a visible line, progressive lenses with a gradual transition, or monovision contact lenses correcting one eye for distance and one for near. Progressives require an adaptation period of a week or two and benefit from accurate fitting heights.

Lens materials and coatings

Standard plastic lenses are inexpensive and thicker in stronger prescriptions. Polycarbonate and Trivex are impact-resistant and lighter, and are the sensible choice for children, sports and safety eyewear. High-index materials are thinner and lighter for strong prescriptions, at higher cost.

Anti-reflective coating reduces glare and reflections and genuinely improves night driving and screen use. Scratch-resistant coating is standard on most lenses. UV protection matters and is built into many materials.

Blue-light filtering coatings are heavily marketed; evidence for meaningful benefit on eye strain or sleep from spectacle coatings specifically remains limited, and reviews have generally found modest or unclear effects. Screen-related discomfort is more often addressed by breaks, blink rate and appropriate correction.

Contact lens prescriptions differ

A contact lens prescription is not the same document as a spectacle prescription and cannot simply be substituted. Contacts sit on the eye rather than roughly 12 mm in front, so powers differ for stronger corrections, and the prescription also specifies base curve, diameter, material and brand.

Contact lens prescriptions require a fitting and typically expire after a year in the US. Wearing lenses beyond their replacement schedule or sleeping in lenses not approved for it raises infection risk substantially.

How often to be examined

Adults with no problems are commonly advised to have eye examinations every one to two years, more often with diabetes, glaucoma risk, high prescriptions or a family history of eye disease. Children benefit from early screening since uncorrected refractive error affects learning.

An eye examination checks more than refraction: intraocular pressure, retinal health, and signs of systemic conditions including diabetes and hypertension are often first detected in the eye.