Understanding Your Vision: Glasses Prescription, 20/20 Vision & Eye Strain | Yamout Optical Center

Understanding Your Vision

A patient-friendly guide from Yamout Optical Center (Since 1978): learn how glasses prescriptions work, what 20/20 vision really means, why the same prescription can feel different, and why eye strain or headaches may be related to vision.

YOC • Patient Guide
All
Basics
Comfort
Most common

What Is a Glasses Prescription? (SPH, CYL, AXIS, ADD Explained)

Understand your Rx in simple words — what each number means and why accuracy matters.
Basics • Core

A glasses prescription (Rx) is a set of numbers that tells us how to focus light correctly on your retina. It is not “one number”—it is a combination of measurements that must match your eyes and your daily needs.

SPH (Sphere)

SPH is the main power that corrects myopia (minus/−) or hyperopia (plus/+). The higher the number, the stronger the correction.

CYL (Cylinder)

CYL corrects astigmatism. Astigmatism means the eye is not perfectly round, so light does not focus evenly. CYL is written with a power and needs an axis to tell the direction.

AXIS

AXIS is a direction (0–180 degrees). It is only meaningful when CYL exists. A small axis change can affect clarity, comfort, and headaches—especially in stronger astigmatism.

ADD (Add Power)

ADD is extra near power for reading (presbyopia). It is used for bifocals and progressives. Most people start needing ADD in the early-to-mid 40s, but it varies.

Other common items
  • PD (Pupillary Distance): helps center lenses correctly for comfort and best vision.
  • Prism: used for alignment issues and must be measured precisely.
Why accuracy matters: Two prescriptions can look “similar” but feel very different if axis, PD, or lens design changes.
Why is my SPH negative?
Negative SPH means myopia (difficulty seeing far). Minus power moves focus onto the retina.
If I have no CYL, do I have astigmatism?
Usually no (or very small). If CYL is blank/0.00, it is not being prescribed.
Can I use someone else’s glasses if the numbers are close?
Not recommended. Small differences (axis/PD) can cause strain, dizziness, or blur.
Does ADD mean my eyes are “weak”?
No. ADD is a normal age-related focusing change for near tasks (presbyopia).

Myopia, Hyperopia, Astigmatism – Simple Explanation

The 3 main reasons people need glasses — explained without complicated science.
Basics • Core

Most prescriptions correct one (or more) of these conditions. Knowing which one you have helps you understand your vision.

Myopia (Nearsightedness)

Distant objects are blurry because the eye focuses light in front of the retina. Glasses use minus (−) power to move focus back.

Hyperopia (Farsightedness)

Light focuses behind the retina. Near tasks may be tiring and distance can also be unclear, especially with age. Glasses use plus (+) power.

Astigmatism

The eye shape is not perfectly round, creating blur/ghosting. It is corrected with CYL + AXIS.

Common misunderstanding: Astigmatism is not a disease. It’s very common and treatable.
Can I have myopia and astigmatism?
Yes. Many people have minus SPH plus CYL.
Does astigmatism always need correction?
Not always. Small amounts may not require correction if you are comfortable.
Why does blur change during the day?
Dryness, fatigue, and screen use can make vision fluctuate.
Is hyperopia only for older people?
No. It can exist at any age; younger people may strain to compensate.

20/20 Vision: What It Means—and What It Doesn’t

You can see 20/20 and still have discomfort, glare, fatigue, or headaches.
Comfort • Core

20/20 measures sharpness on an eye chart, not the full quality of your vision. Comfort and function depend on more than one number.

What 20/20 measures

It measures how small letters you can read at a specific distance (visual acuity).

What it does NOT measure
  • Glare/halos (night driving)
  • Dry eye and fluctuating blur
  • Eye coordination (binocular vision)
  • Focusing stamina (screen/reading fatigue)
  • Contrast sensitivity (seeing in dim light)
Important: If you have 20/20 but symptoms persist (headache, strain, halos, double vision), you still need a full professional evaluation.
Does 20/20 mean perfect vision?
No. It only means normal sharpness in one test condition.
Why is night vision worse even if I see 20/20?
Night vision depends on glare, pupil size, dryness, and lens clarity—not only acuity.
Can coating improve comfort?
Yes. Good anti-reflective coating can reduce glare and improve contrast.
Should I still update my glasses?
If symptoms exist or needs changed (screens/reading), an update may help.

Why Two People With the Same Prescription See Differently

Vision is not only numbers — it is optics + eye health + fitting + brain adaptation.
Comfort • Core

Two people can have identical SPH/CYL/AXIS and still report different clarity or comfort. This is normal.

Main reasons
  • Different PD and centering: the same Rx needs correct optical alignment for each person.
  • Different lens designs: aspheric vs spherical, single vision vs progressive.
  • Different frame sizes: larger frames can increase distortions and thickness effects.
  • Different coatings/material: glare control and contrast vary.
  • Different eye conditions: dryness, early cataract, corneal irregularities.
  • Different adaptation: some adapt quickly, others need time.
Clinical point: Prescription + lens selection + fitting = final result.
Can the same Rx feel different in a new frame?
Yes. Fit, tilt, and lens position can change perception.
Why do progressives feel different between brands?
Design differences change corridor and clarity zones.
Is adaptation time real?
Yes. Especially with astigmatism changes or progressives.
What if my new glasses feel wrong?
Return for verification: lens check, fitting, and re-evaluation if needed.

Why Your Old Glasses No Longer Feel Comfortable

If glasses used to feel fine but now feel wrong, there is always a reason.
Comfort • Core

Many patients say: “These glasses were perfect, but now I feel strain or blur.” This is common. The reason can be the eyes, the lenses, the frame fit, or your daily tasks.

Common causes
  • Your eyes changed: focusing ability reduces with age.
  • Lens scratches/coating wear: glare increases and contrast drops.
  • Frame fit changed: pads loosen, frame sits lower, temples bend.
  • More screen time: visual demands changed.
  • Dry eye: causes fluctuating blur and fatigue.
Warning sign: If discomfort is sudden/severe, or you have double vision, flashes, or major blur—seek professional care.
Can a small Rx change cause big discomfort?
Yes. Small axis changes or near support needs can strongly affect comfort.
Can frame position cause blur?
Yes. Especially with progressives if you look through the wrong zone.
Should I replace lenses if Rx is unchanged?
If coatings are damaged or scratches reduce clarity, replacing lenses can help.
Why do I feel tired after reading now?
Often presbyopia, dry eye, or focusing fatigue. A recheck identifies the cause.

Why Headaches Can Be Related to Vision

Many headaches come from eye strain, focusing effort, dryness, or binocular issues.
Comfort

Not all headaches are vision-related—but many are. Eye strain headaches often worsen with reading, computer use, or long concentration.

Vision-related causes
  • Uncorrected prescription (especially astigmatism)
  • Incorrect PD/centering or lens alignment issues
  • Presbyopia beginning (near work becomes tiring)
  • Binocular vision problems (eyes not teaming comfortably)
  • Dry eye and screen fatigue
  • Glare (poor coatings, night driving discomfort)
Seek urgent medical care if headache is sudden/severe, with neurological symptoms, vision loss, or unusual weakness. This page is for common eye-strain causes, not emergencies.
Helpful tip: If headaches appear mainly during near work, a professional eye exam and correct lens design often helps.
How do I know it’s from vision?
If headaches worsen with screens/reading and improve with rest, vision is a strong possibility.
Can blue light lenses stop headaches?
Sometimes comfort improves, but main causes are often Rx accuracy, dryness, and focusing fatigue.
Does astigmatism cause headaches?
Yes—especially if cylinder/axis is not correct or you strain to compensate.
What is the best solution?
A full eye exam, correct Rx, correct PD/centering, and a lens design matching your daily tasks.
Medical note: This content is general education and does not replace a professional eye examination. For personal advice, schedule a check at Yamout Optical Center.