How to Read Your Eye Prescription — And What You Should Know About Digital Eye Strain in 2026

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

You just picked up your glasses prescription. There is a row of numbers, abbreviations in Latin, plus and minus signs — and most people have no idea what any of it means. At Charm Optical on Ellerslie Road in south Edmonton, we walk through prescriptions with patients from Windermere, Heritage Valley, Rutherford, Summerside, Chappelle, and across the region every day. This is the complete guide — prescription explained, plus what the research actually says about digital eye strain and blue cut lenses.

Eye care professionals worldwide use Latin abbreviations. Your two eyes are always listed separately:

It is completely normal — and common — for your OD and OS values to differ. Each eye is independently shaped and develops independently. Having one eye with stronger correction than the other is the norm, not the exception.

These are measured in diopters (D) — the unit of optical lens power. Here is what each value means:

1. SPH — Sphere

The primary correction your eyes need. The sign tells you which refractive error you have:

•       Minus (−) values  indicate myopia (nearsightedness). Distant objects are blurry. Higher absolute number = more correction needed

•       Plus (+) values  indicate hyperopia (farsightedness). Near focus may be difficult. Higher number = more correction needed

2. CYL — Cylinder

If there is a number in the CYL column, you have astigmatism — a condition where the cornea is shaped more like a rugby ball than a perfect sphere, causing blurred or distorted vision at all distances, not just near or far. If the CYL column is blank or 0.00, you have no measurable astigmatism. A 2023 PMC systematic review found astigmatism affects approximately 40.4% of adults globally — making it one of the most common refractive conditions worldwide.

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

3. AXIS

AXIS always appears with CYL — never alone. It is a number from 1 to 180 (degrees) that tells the lens lab which direction to orient the astigmatism correction on your lens. It is a map coordinate, not a severity measure. A misalignment of just 5 degrees can reduce visual clarity by over 10% and cause strain and headaches — which is exactly why in-person prescription measurement matters, and why an accurate PD measurement at fitting is essential.

Source: PMC — Prescribing and fitting spectacles: the role of pupillary distance and the optical centre (2024)

4. ADD — Addition

ADD only appears on prescriptions for progressive or bifocal lenses. It is extra magnification power for near vision — always a positive number, typically +0.75 to +3.00 D. If you're over 40 and finding close vision harder, this is the presbyopia correction. It increases gradually until around age 60 when the eye's natural lens has fully stiffened.

5. PD — Pupillary Distance

PD is the distance in millimetres between the centres of your two pupils — typically 58-65 mm in adults. It may not be printed on your prescription but is essential for making glasses. The PMC clinical guide on spectacle fitting (2024) states: "If the pupillary distance is measured incorrectly, the optical centre will be incorrectly set... which can only be rectified by remaking the spectacles." Always have PD measured in person.

Sources: PMC — Prescribing and fitting spectacles: role of pupillary distance (2024) | PMC — Epidemiology and Burden of Astigmatism (2023) | American Academy of Ophthalmology (2025)

This is one of the most common misunderstandings at Charm Optical. A glasses prescription and a contact lens prescription are separate documents, even if your underlying vision is identical. Glasses sit 12-14 mm from your cornea. Contact lenses sit directly on the corneal surface. This physical difference changes the effective optical power needed.

Contact lens prescriptions also include base curve (BC) and diameter (DIA) values — measurements of your cornea's curvature and the lens size needed for a proper fit. These values come from a contact lens trial fitting and are not part of a glasses prescription. Never order contact lenses using only your glasses prescription.

How Common Is It?

Very common and growing. A 2024 PMC meta-analysis analyzing 103 studies across 66,577 participants from over 20 countries found digital eye strain (Computer Vision Syndrome) affects approximately 66-69% of regular computer users globally. A 2024 systematic review highlighted in the PMC Computer Vision Syndrome comprehensive literature review found 74% of participants experienced CVS symptoms — substantially higher than pre-pandemic rates.

What Actually Causes It?

The evidence points to two primary mechanisms, and neither is simply 'blue light':

•       Reduced blink rate  — at screens, people blink only 3-7 times per minute versus the normal 18-22 times per minute according to the PMC Digital Eye Strain: Updated Perspectives review (2024). This dramatically reduces tear film distribution, causing dryness, burning, and foreign body sensation

•       Accommodation and vergence stress  — your focusing muscles continuously micro-adjust to maintain sharp focus on pixels, which are less well-defined than printed text. Prolonged ciliary muscle contraction causes fatigue, blurred vision, and headaches

Poor ergonomics, low ambient humidity (relevant in Edmonton's dry climate year-round), glare, and insufficient breaks all compound these effects.

What Does the Evidence Say About Blue Light Specifically?

The evidence is mixed — and we think it's important to be honest about that. A systematic review in Advances in Ophthalmology Practice and Research (ScienceDirect, 2025) found that "blue-blocking filters do not appear to prevent DES" based on available studies, with low to moderate quality evidence. The American Academy of Ophthalmology states there is no scientific evidence that screen light causes permanent eye damage.

However, a PMC pilot study on blue light filtering lenses in radiology residents found 68.8% of CVS symptoms were reduced when participants wore blue light filtering lenses versus non-filtering lenses. Blue light is also associated with melatonin suppression — filtering it in evening hours may support better sleep quality.

The honest summary: the most evidence-backed interventions for digital eye strain are the 20-20-20 rule, proper screen ergonomics, and conscious blinking. Blue cut lenses are a useful addition for many people — but not a substitute for eye hygiene habits.

Sources: PMC — Computer vision syndrome: comprehensive literature review (2025) | PMC — Digital Eye Strain: Updated Perspectives (2024) | PMC — Blue light filtering glasses and CVS in radiology residents | ScienceDirect — Digital eye strain systematic review (2025)

At Charm Optical, our blue light filtering lenses are called Digital+. They can be added to any prescription — single vision, bifocal, or progressive — as a coating that filters high-energy blue light and reduces screen glare. They look like regular clear lenses with a subtle green surface reflection. No yellow tint, no change to colour perception.

The key advantage at Charm Optical: single vision Digital+ lenses are available the same day. Walk in with a current prescription and you could leave wearing them that same visit. Progressive and bifocal blue cut lenses take approximately two weeks. Visit our Digital+ Blue Cut Lenses page or see our same-day glasses service for all same-day options.

At Charm Optical, 5035 Ellerslie Road SW, Edmonton, AB T6X 1X2, we help patients understand their prescriptions and find the right lens options every day. 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.

Book a comprehensive eye exam for a current prescription. Browse our glasses collection (single vision from $99, progressives from $350 — see our glasses deals). Add Digital+ blue cut lenses to any prescription. We direct bill all major Alberta insurance providers including Alberta Blue Cross, Canada Life, ASEBP, and 30+ more.

My prescription has a minus sign — does that mean my vision is bad?

No. Minus signs simply mean myopia (nearsightedness). A -1.50 prescription is mild. Even prescriptions like -5.00 or -7.00 correct fully to clear vision with the right lenses — they are not 'bad eyes,' just eyes with a longer-than-average shape. Glasses and contacts correct it completely.

What does it mean if my CYL column is blank?

It means you have no measurable astigmatism in that eye. Your cornea is spherically symmetric and only a sphere correction is needed. Most adults over 30 have at least some mild astigmatism, even if it's minor enough not to warrant correction.

Can I use my glasses prescription to order contact lenses?

No. Glasses and contact lens prescriptions are different documents. Contact lens prescriptions include base curve and diameter values measured specifically for lens-on-cornea fitting. These require a contact lens trial fitting and cannot be derived from a glasses prescription. Never order contact lenses using your glasses prescription alone.

Do blue light glasses actually work?

The evidence is genuinely mixed. Some studies show meaningful symptom reduction in digital eye strain with blue light filtering lenses. Others find no significant difference. What is clear is that digital eye strain itself is real and affects approximately 66-69% of regular screen users. Blue cut lenses are a useful tool for many people — especially combined with the 20-20-20 rule and proper ergonomics. At Charm Optical, our Digital+ lenses are clear, comfortable, and available same-day for single vision.

How often should I update my glasses prescription?

The Canadian Association of Optometrists recommends comprehensive eye exams every 1-2 years for healthy adults aged 20-64, and annually for adults 65 and older. Wearing an outdated prescription is a surprisingly common cause of headaches and eye strain. If you are struggling with screen fatigue or visual discomfort, check when you last had an exam.

Is my eye exam covered by Alberta Health Care?

Under AHCIP, children under 19 are covered for one comprehensive eye exam per benefit year (July 1 to June 30) with a co-pay of approximately $30 at most clinics. Adults 65 and over are covered annually. Adults aged 19-64 should check their employer benefits. We direct bill for all major providers at Charm Optical.

• PMC — Computer vision syndrome: a comprehensive literature review (2025)

• PMC — Digital Eye Strain: Updated Perspectives (2024)

• PMC — Digital Eye Strain: A Comprehensive Review

• PMC — Impact of blue light filtering glasses on CVS in radiology residents

• PMC — Visual impact of smartphones: narrative review of ocular changes (2025)

• ScienceDirect — Digital eye strain in young screen users: systematic review (2025)

• PMC — Prescribing and fitting spectacles: the role of pupillary distance (2024)

• PMC — Epidemiology and Burden of Astigmatism: Systematic Literature Review (2023)

• American Academy of Ophthalmology — Eye prescription and astigmatism guide (2025)

• Canadian Association of Optometrists — Eye exam frequency guidelines

• Charm Optical — Digital+ Blue Cut Lenses (same-day)

• Charm Optical — Same-Day Glasses Service

• Charm Optical — Glasses Collection

• Charm Optical — Glasses Deals

• Charm Optical — Eye Exam and Services

• Charm Optical — Insurance Providers

• Charm Optical — Book an Eye Exam