What Is Astigmatism? How It Affects Your Vision, Your Glasses, and Your Contacts

By Navid H., Licensed Optician  |  Charm Optical  |  5035 Ellerslie Road SW, Edmonton, AB T6X 1X2  |  March 2026  |  11 min read

You've just picked up your glasses prescription and you're staring at a line of numbers: SPH, CYL, AXIS. You understand SPH — that's the nearsighted or farsighted part. But CYL and AXIS? Most people have no idea what those mean, even after years of wearing glasses.

Those two numbers describe astigmatism — one of the most common refractive conditions in the world, and one of the most misunderstood. At Charm Optical on Ellerslie Road we explain astigmatism to patients from across south Edmonton every single week — from Windermere, Heritage Valley, Rutherford, Summerside, Chappelle, and beyond. This is the post we wish everyone had read before their first eye exam.

Source: PMC — Epidemiology and Burden of Astigmatism: A Systematic Literature Review (Optometry and Vision Science, 2023)

Astigmatism is not a disease. It is not an eye condition that gets worse over time on its own, and it is not caused by reading in dim light or sitting too close to a screen. Those are myths. Astigmatism is simply a refractive error — a variation in the shape of your cornea (the clear front surface of your eye) or your eye's internal lens.

A normal cornea is shaped like a perfectly round basketball — the same curvature in every direction. In a cornea with astigmatism, the shape is more like a rugby ball or an egg — steeper in one direction, flatter in another. This means light entering your eye is refracted differently depending on which angle it comes from, and instead of converging at one single sharp point on your retina, it creates two focal points. The result: blurred or distorted vision at all distances, not just at one.

The American Academy of Ophthalmology puts it simply: "Astigmatism is an imperfection in the normal curvature of your eye's cornea or lens." That's all it is. It is extremely common, completely correctable with the right lenses, and in the vast majority of cases poses no risk to your long-term eye health.

The PMC systematic literature review on astigmatism epidemiology — which analyzed 125 studies from 6,804 initial citations published in Optometry and Vision Science (2023) — found that prevalence in the general population varies widely from 8% to 62%, with higher rates in older adults. A pooled meta-analysis estimate puts the adult prevalence at approximately 40% globally.

Sources: PMC — Epidemiology and Burden of Astigmatism: Systematic Literature Review (2023) | ScienceDirect — International trends in silicone hydrogel contact lens prescribing (2024)

From a Canadian lens prescribing perspective, 32% of all soft contact lens fits are toric lenses — meaning nearly one in three people fitted with soft contacts in Canada needs astigmatism correction. If you have astigmatism, you are far from alone.

In most cases astigmatism is simply inherited — you are born with a cornea that isn't perfectly round, just as your nose or ears have their own distinct shape. The American Academy of Ophthalmology confirms: "Doctors don't know why the shape of the cornea or lens varies from person to person. But they do know the risk of getting astigmatism is inherited."

Other factors that can cause or worsen astigmatism include:

•       Age  — the type of astigmatism typically shifts with age. With-the-rule astigmatism (steeper vertically) is more common in people under 40. Against-the-rule astigmatism (steeper horizontally) increases with age, according to the PMC systematic review.

•       Eye injuries or surgery  — trauma that alters corneal shape can create or change astigmatism

•       Keratoconus  — a progressive condition where the cornea gradually thins and takes on a cone shape, creating significant irregular astigmatism. This is more serious than typical astigmatism and usually requires specialized contact lens fitting rather than standard glasses or toric lenses

•       Eye rubbing  — chronic, forceful eye rubbing has been associated with corneal shape changes in some individuals

What does not cause astigmatism: reading in dim light, too much screen time, or sitting close to a TV. These are persistent myths with no clinical evidence behind them.

The PMC systematic literature review documented the real impact of uncorrected astigmatism on quality of life and daily function. People with uncorrected astigmatism experience:

•       Blurry or distorted vision at all distances  — not just far or near, but both. This distinguishes astigmatism from simple myopia or hyperopia

•       Increased glare  — 53 to 77% of astigmatic patients in studies reported significant glare sensitivity

•       Halos around lights  — reported by 28 to 80% of astigmatic patients

•       Night-time driving difficulties  — 66% of astigmatic patients experienced difficulty driving at night in the PMC review data. Oncoming headlights scatter into streaks or star-burst patterns

•       Eye strain and headaches  — particularly after prolonged near work, reading, or screen use. The eyes continuously attempt to compensate for the unfocused image

•       Squinting  — squinting physically elongates the eye opening, which acts as a crude pinhole and temporarily sharpens vision

•       Fatigue  — the visual system works harder to form a clear image, which is tiring over a full day

Critically, the PMC review found that astigmatic patients perform vision-related tasks 9% slower with 1 dioptre of uncorrected astigmatism and 29% slower with 2 dioptres — making errors at significantly higher rates. This is why correcting even moderate astigmatism has a measurable effect on daily function, not just visual comfort.

This is the part nobody explains clearly. When you get your glasses or contact lens prescription, it contains at minimum three numbers per eye: SPH, CYL, and AXIS. Here is exactly what each one means:

Sources: American Academy of Ophthalmology — Astigmatism prescription guide | Discovery Eye Foundation — Understanding Your Eye Prescription | PMC — Epidemiology and Burden of Astigmatism (2023)

A Real Prescription Example

Here is what a typical prescription with astigmatism looks like:

In this example, both eyes are nearsighted (negative SPH) and both have astigmatism (CYL values present). The AXIS values differ between eyes — 175 and 90 — which is completely normal. As the American Academy of Ophthalmology explains, the axis is a map coordinate that tells the lab where on your cornea the steepest curve sits, so the cylindrical correction can be oriented to exactly counteract it.

Glasses correct astigmatism using cylindrical (toric) lenses — lenses that are curved differently in one axis versus another, creating different refracting power at different meridians. Unlike a standard spherical lens, which has the same curve in every direction, a cylindrical lens has one flat meridian and one curved meridian.

When the lab makes your lenses, they grind the cylindrical component at precisely the angle specified by your AXIS value. The goal is to add exactly the right compensating curve to neutralize your cornea's irregular shape. The result is that light from all directions converges at one point on your retina instead of two.

For most people with typical regular astigmatism, standard prescription glasses with toric lenses work perfectly well. Frame choice matters too — very large or curved wrap-around frames can tilt the lens away from the correct optical angle, slightly compromising the astigmatism correction. Our licensed optician at Charm Optical checks lens tilt and positioning at every fitting. See our full glasses collection or ask about our glasses deals — single vision from $99, progressive from $350.

Standard round contact lenses cannot correct astigmatism — they rotate freely on the eye with each blink, and a spherical lens has no directional correction to offer. Astigmatism requires toric contact lenses — lenses with different powers at different meridians, combined with a design that keeps the lens oriented consistently on your eye.

How Toric Contact Lenses Stay in Place

The central challenge with toric soft contacts is lens rotation. If the lens rotates 10-15 degrees from the correct axis orientation, the astigmatism correction shifts and vision blurs. Toric lens manufacturers solve this through stabilization designs:

•       Prism ballast  — a small amount of additional thickness at the bottom of the lens, like a pendulum weight, uses gravity and eyelid pressure to keep the lens consistently oriented

•       Dual thin zones  — thinner zones at the top and bottom of the lens, with thicker zones on each side, so the eyelids apply pressure that consistently aligns the lens

•       Accelerated stabilization design  — used by Acuvue toric lenses, this design responds to eyelid movement to quickly re-orient after blinking

A PMC real-world study on toric soft contact lens effectiveness published in 2023 — covering 384 astigmatic patients across cylinder groups from -0.50 to >-2.00 D — confirmed that toric soft contact lenses achieve successful fitting and visual acuity correction across all astigmatism severities, with scribe marks showing lens orientation fell within 0-10 degrees of the target axis in properly fitted lenses.

Source: PMC — Effectiveness of toric soft contact lenses for vision correction in patients with different degrees of astigmatism: a real-world study (2023)

Toric Contact Lenses Available at Charm Optical

All toric contact lens fittings at Charm Optical include a trial lens assessment to confirm the lens sits at the correct axis orientation on your specific eye. This step is not optional — it is the difference between a lens that works and one that blurs at the worst moments. Contact lenses are a medical device that requires proper prescription and fitting, not an off-the-shelf product.

Most people with astigmatism have regular astigmatism — the cornea is consistently steeper along one meridian, like a rugby ball. This corrects straightforwardly with standard cylindrical lenses in glasses or toric contact lenses.

A smaller number of people have irregular astigmatism — where the corneal shape is inconsistently curved and cannot be fully corrected with standard spherocylindrical lenses. The most common cause is keratoconus — a condition where the cornea progressively thins and bulges forward into a cone shape. In early keratoconus, glasses or soft toric lenses can provide acceptable correction. As it progresses, specialized contact lens designs — rigid gas-permeable lenses, hybrid lenses (rigid center with soft skirt), or scleral lenses — are needed to vault over the irregular surface and provide clear vision.

If you experience progressive worsening of your astigmatism prescription, halos, ghosting, or visual distortion that doesn't fully correct with glasses, this warrants assessment during a comprehensive eye exam. Keratoconus detected early can be slowed significantly with corneal cross-linking treatment.

Astigmatism is diagnosed during a comprehensive eye exam. The process involves:

•       Autorefractor  — an instrument that shines light into the eye and measures how it reflects back, giving an objective starting estimate of your sphere, cylinder, and axis values

•       Keratometry  — measures the curvature of your cornea at specific meridians, directly detecting the amount and orientation of corneal astigmatism

•       Phoropter refinement  — the classic 'which is better, one or two?' process, where the optician fine-tunes both the cylinder power and axis until you confirm the clearest view. This subjective refinement is critical — objective instruments give a starting point but your feedback determines the final prescription

•       Corneal topography (if indicated)  — a detailed map of the entire corneal surface, identifying irregular astigmatism patterns that keratometry alone would miss. Used when keratoconus or other corneal conditions are suspected

Alberta Health Care (AHCIP) covers comprehensive eye exams annually for all children under 19 and adults 65 and older. Adults 19-64 should check their employer benefits — most group health plans include an annual exam. If you have astigmatism and haven't had a prescription update in over 12 months, come in — it may have changed.

We diagnose and correct astigmatism every day at Charm Optical, 5035 Ellerslie Road SW, Edmonton, AB T6X 1X2. We serve patients from Ellerslie, Windermere, Heritage Valley, Summerside, Rutherford, Chappelle, Cavanagh, Ambleside, Allard, Callaghan, Blackmud Creek, The Meadows, Walker, Twin Brooks, Mill Woods, Beaumont, Leduc, Nisku, and Sherwood Park.

Whether you need an updated prescription, a first-time toric contact lens fitting, or you've been told you have astigmatism but nobody fully explained it, come in. We carry a full range of toric contact lenses from Acuvue, Dailies (Alcon), CooperVision, Bausch + Lomb, and Alcon, plus prescription toric glasses across all our frame brands. We do direct billing for all major Alberta insurance providers including Alberta Blue Cross, Canada Life, ASEBP, and more.

Is astigmatism serious? Will it get worse?

For most people, regular astigmatism is stable and not progressive. It is a shape characteristic of your cornea, not a disease. It can be fully corrected with the right lenses. Some change over time is normal — particularly the shift from with-the-rule to against-the-rule astigmatism as you age — which is why annual eye exams are recommended. If your astigmatism is changing rapidly, your optician will assess for keratoconus or other underlying causes.

Can I wear regular contact lenses if I have astigmatism?

In theory, a standard spherical contact lens can mask mild astigmatism (below about -0.75 CYL) by forming a smooth liquid lens over the cornea. In practice, most people with CYL of -0.75 or higher will see noticeably better with toric lenses. If you have tried regular contacts and found them blurry or unstable, toric lenses are almost certainly the answer. A fitting appointment at Charm Optical will confirm this.

Why does my prescription say different axis values for each eye?

Completely normal. Each eye has its own independent shape, and the orientation of astigmatism rarely matches between eyes. It would be unusual — not reassuring — if both eyes had identical axis values. The right eye is fitted and corrected entirely independently of the left.

My glasses prescription has a CYL value but my contacts prescription doesn't — why?

Contact lens prescriptions are sometimes written without a cylinder correction for mild astigmatism, particularly if the CYL is below -0.75 D. This is a clinical judgment call: if the astigmatism is mild enough that a spherical lens provides adequate vision, the optician may choose simplicity over marginal improvement. If you feel your contacts are blurrier than your glasses, mention this at your next fitting — toric lenses may give you meaningfully better vision.

Do I need a separate prescription for toric contact lenses vs my glasses?

Yes. A glasses prescription and a contact lens prescription are different documents — even if you have the same underlying astigmatism. Contact lenses sit directly on the cornea, while glasses sit 12-14mm away from the eye, and this distance changes the effective power needed. A contact lens fitting also measures corneal curvature (base curve) and lens diameter, which are not part of a glasses prescription. Never order toric contacts using only your glasses prescription.

• PMC — Epidemiology and Burden of Astigmatism: A Systematic Literature Review (Optometry and Vision Science, 2023)

• PMC — Effectiveness of toric soft contact lenses for vision correction in patients with different degrees of astigmatism: a real-world study (2023)

• ScienceDirect — International trends in prescribing silicone hydrogel contact lenses for daily wear 2000-2023 (2024)

• American Academy of Ophthalmology — Astigmatism: Causes, Diagnosis, Treatment (2025)

• PMC — Current perspectives in the management of keratoconus with contact lenses

• PMC — Update on Contact Lens Treatment of Keratoconus

• Canadian Association of Optometrists — Eye health and refractive error guidance

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• Charm Optical — Eye Exam and Services

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• Charm Optical — Bausch + Lomb

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