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Spectacle Prescription Guide

What's the key thing to understand about Reading Your Prescription?

SPH (sphere): the main correction power in dioptres. Negative = short-sighted (myopia) — distant objects blurry. Positive = long-sighted (hyperopia) — close objects blurry (and sometimes distant too). +1.00 to +3.00: mild hyperopia. +3.00 to +6.00: moderate. Above +6.00: high. −1.00 to −3.00: mild myopia. −3.00 to −6.00: moderate. Below −6.00: high myopia. CYL (cylinder): corrects astigmatism — the cornea or lens is not perfectly spherical. Negative CYL is most common in UK/European notation. Some prescriptions also include an AXIS value alongside CYL, which specifies the orientation of the astigmatism correction in degrees.

What should I know about Spherical Equivalent?

Spherical equivalent = SPH + CYL/2. Used for contact lens prescriptions as a single power (toric contacts for astigmatism correction are a separate matter). Example: SPH −2.50, CYL −0.75, AXIS 90. Spherical equivalent = −2.50 + (−0.75/2) = −2.50 − 0.375 = −2.875 D. A −3.00 contact lens would be prescribed (rounded to nearest 0.25). For astigmatism above approximately −1.00 CYL, toric contact lenses are usually recommended instead of the spherical equivalent approximation.

What's the difference between Glasses and Contact Lens Prescription?

A spectacle prescription CANNOT be directly used for contact lenses — they sit on the eye at a different vertex distance (distance from lens to cornea). For powers above ±4.00D, the contact lens power differs from the spectacle prescription by a small but clinically significant amount. Vertex distance correction: contact lens power = spectacle power / (1 − vertex distance × spectacle power). Vertex distance typically 12-14mm. At −8.00D glasses, the contact lens power = −8/(1−0.012×(−8)) = −8/1.096 ≈ −7.30D — a meaningful difference at this power that your optician accounts for automatically when fitting contacts. Managing dryness or irritation between exams is worth doing regardless of prescription. CorneaCare makes eye-care products developed with ophthalmologists specifically for contact lens wearers and dry eye sufferers.

What's the key thing to understand about Astigmatism?

Astigmatism occurs when the cornea or lens is slightly oval rather than perfectly spherical — like the surface of an egg rather than a ball. This causes images to focus on two different planes rather than one, creating a blurry or distorted image at all distances. CYL: the difference in power between the two meridians. Up to −0.75D: mild, may not require correction. −0.75 to −1.50D: moderate astigmatism. Above −1.50D: more significant. AXIS: the angle of the most curved meridian, measured in degrees from 0° to 180° — for example 90° (vertical) or 180° (horizontal).

What should I know about Pupillary Distance (PD)?

Your prescription tells an optician the power your lenses need, but pupillary distance (PD) — the measurement in millimetres between the centres of your pupils — determines where that power is correctly centred in the lens. Get PD wrong and even a perfectly accurate prescription can cause eye strain, headaches or subtle double vision, because you're looking through the wrong part of the lens rather than its optical centre. Adult PD typically falls between about 54 and 74mm, and most people have a slightly different near PD (for reading distance) and distance PD, since the eyes converge slightly more for close-up tasks. PD isn't usually written on prescriptions from an eye test because it's measured separately, often at the point of ordering glasses, using a ruler, pupillometer or corneal reflection device. If you're ordering glasses online you'll usually need to supply this number yourself or have it measured in-store first; getting it measured professionally is more reliable than a self-measurement guide.

What should I know about Presbyopia and Reading Additions?

Presbyopia is the gradual, universal loss of near-focusing ability that begins in nearly everyone around age 40-45, caused by the eye's natural lens stiffening and losing flexibility with age — it is unrelated to short- or long-sightedness, though it occurs alongside them. On a prescription this is corrected with a reading "ADD" (addition) power, a plus value added to the distance prescription to boost near-vision strength, typically ranging from +0.75D in the mid-40s up to +2.50D or +3.00D by the mid-60s, after which it generally stops increasing. The ADD is always written as a positive number regardless of whether the distance SPH is positive or negative, because the eye needs extra plus power specifically for close work. Reading glasses apply the ADD alone; varifocals and bifocals blend it with the distance prescription so one pair of glasses handles both distances. Multifocal contact lenses work the same way, combining distance and near power in a single lens. Anyone noticing they hold books or phones further away, or need brighter light to read small print, should have an eye test — presbyopia is not preventable, but it is easily corrected once measured.

What should I know about When to Get an Eye Test?

In the UK, opticians generally recommend an eye test at least every two years for adults, and annually for children, people over 70, and anyone with diabetes (who are entitled to free NHS tests). More frequent checks are advised for people with a family history of glaucoma, high prescriptions, or certain health conditions like diabetes and high blood pressure that can affect the retina. A sudden change in vision, new floaters, flashes of light, or a shadow across the visual field should prompt an urgent appointment rather than waiting for a routine test, since these can signal retinal detachment or other emergencies. Children's prescriptions can change quickly during growth spurts, so myopia (short-sightedness) in particular is often monitored more closely at this stage, including with management options such as orthokeratology or specific contact lenses aimed at slowing progression, which a standard glasses prescription doesn't address.

Astigmatism Axis: Why the Number After 'x' Matters

On a prescription with a cylinder (CYL) value, the axis — written as a number from 0 to 180 after an 'x', such as CYL -1.25 x 90 — specifies the orientation of the astigmatism correction in degrees, not a strength. Two prescriptions with an identical cylinder power but different axis values correct astigmatism in different orientations of the eye and are not interchangeable, which is why axis is always quoted alongside cylinder power on a valid glasses or contact lens prescription rather than cylinder power being used on its own.

Worked Example: Calculating Spherical Equivalent From a Full Prescription

Spherical equivalent (SE) is a single number that approximates the overall power of a prescription that includes astigmatism correction, calculated as SE = Sphere + (Cylinder / 2). Take a prescription written as Sphere -2.00 DS, Cylinder -1.50 DC. Half of -1.50 is -0.75, so SE = -2.00 + (-0.75) = -2.75 D. This -2.75 D figure is useful as a rough, single-number description of overall short-sightedness when comparing two prescriptions at a glance, or for situations such as rough contact lens estimates or safety-eyewear ordering where a toric (astigmatism-correcting) lens is not available and only a spherical lens can be supplied. It is not a substitute for the full prescription: someone with -2.75 DS and no astigmatism sees differently, especially at distance and in low light, from someone whose -2.75 D is a spherical equivalent built from a large cylinder component. Opticians always dispense glasses using the full sphere, cylinder and axis rather than the spherical equivalent alone.

Lens Power Ranges and What They Typically Represent

Prescriptions are grouped informally into bands of severity, which can help put a number in context, although an optometrist's assessment of the whole eye always matters more than the number alone.

High myopia in particular is worth flagging because, independent of whether vision feels sharp with correction, it carries a somewhat increased long-term risk of retinal detachment and other retinal changes, which is one of the reasons optometrists recommend keeping to recommended eye test intervals rather than only testing vision when glasses seem to need updating.