What Happens During an Eye Test — and How Often You Need One
An eye test is not only about whether you need glasses. It is one of the few routine checks where a clinician can look directly at your blood vessels and nerves.
A full examination takes 20 to 30 minutes and is completely painless. Here is what each part is for, so nothing in the room is a surprise.
1. History and symptoms
Questions about your vision, your work, your screen hours, any headaches, plus family history of glaucoma or retinal disease, and general health — particularly diabetes and blood pressure, both of which show up in the eye before they show up anywhere else you can see.
2. Visual acuity — the letter chart
One eye at a time, reading down the chart. This produces the 6/6 or 20/20 figure. It measures how much detail you can resolve — not the health of the eye, and not your prescription.
3. Autorefraction and retinoscopy
The machine you rest your chin on that shows a small image, often a road or a balloon. It gives an objective starting estimate of your prescription in seconds. It is a starting point, never the final number.
4. Subjective refraction — "better with one, or two?"
The part everyone remembers. Lenses are swapped in front of each eye and you compare pairs. There is no wrong answer and "they look the same" is a genuinely useful response — it tells the optometrist they have reached the point where the difference no longer matters, which is exactly what they are looking for.
5. Eye pressure (tonometry)
The brief puff of air. It screens for raised intraocular pressure, the main risk factor for glaucoma — a condition that destroys peripheral vision silently and irreversibly, and which has no symptoms until a great deal has already been lost. The puff is startling and completely harmless.
6. Examining the back of the eye
With a slit lamp, an ophthalmoscope, or a retinal photograph. This is the clinically important part: the retina is the only place in the body where blood vessels and a nerve can be observed directly, without any incision. Diabetic changes, hypertensive changes, glaucoma damage and retinal tears are all found here, frequently in people who had no symptoms and came in for new glasses.
How often should you be tested?
| Who | How often |
|---|---|
| Adults under 40, no symptoms | Every 2 years |
| Adults over 40 | Every 1–2 years |
| Children and teenagers | Every year |
| Diabetes, at any age | Every year, without exception |
| Family history of glaucoma | Every year from age 40 |
How to prepare
- Bring your current glasses, and any older pair you still use.
- Bring your contact lens details if you wear them, and ideally leave them out for a few hours beforehand.
- Bring a list of any medication you take — several affect the eye directly.
- Bring sunglasses in case your pupils are dilated.
- Mention any headaches, double vision, floaters or flashes — especially if they are new. New flashes and floaters together warrant same-day attention.
Pick a branch and a time — you will leave with your prescription and your PD in writing.
Book an eye testFrequently asked questions
How long does an eye test take?
A full examination takes 20 to 30 minutes. Add an extra 20 minutes if your pupils need dilating, plus four to six hours of blurry, light-sensitive vision afterwards — so arrange not to drive.
Does an eye test hurt?
No part of a routine eye test is painful. The puff of air used to check eye pressure is startling for a second, and dilating drops sting slightly for a few moments. Nothing touches the surface of your eye in a standard examination.
How often should I have my eyes tested?
Every two years for healthy adults under 40, annually from 40 onwards, and annually for children, for anyone with diabetes, and for anyone with a family history of glaucoma. See an optometrist immediately if you notice sudden vision loss, new flashes of light, a sudden shower of floaters, or eye pain.
Can an eye test detect other health problems?
Yes, and this is one of its most valuable functions. The retina is the only place a clinician can observe blood vessels and a nerve directly. Diabetic retinopathy, high blood pressure, raised cholesterol and some neurological conditions all leave visible signs there, often before the person has any other symptoms.