Eye test at the ophthalmologist — what is measured, and what it does not show

What a professional eye test covers

Most people mean by 'eye test' a chart on the wall, one eye covered and letters that get smaller and smaller. That is indeed the core of it: determining your visual acuity — in technical terms, the visus. It describes how finely your eye can still perceive two points as separate, once without and once with the best possible correction. This measurement is one of several examinations of the eye.

An eye test at the ophthalmologist's does not stop there, though. As a rule it comprises:

• The conversation first — what has changed, since when, at what distance it bothers you, how you work, read and drive
• Measuring your current glasses, so that we know what you have been seeing with until now
• The measurement by machine (objective refraction) as the baseline value
• The fine-tuning with you (subjective refraction) — 'better like this, or like this?'
• Near vision with a reading chart, from midlife onwards the real everyday question
• The examination of the eye itself at the slit lamp, where required with intraocular pressure measurement and assessment of the optic nerve and retina
• The discussion — findings, prescription, and the question of whether and when a follow-up makes sense

Optician or ophthalmologist — where is the difference?

Opticians measure carefully, with the same classes of instrument and often with a great deal of routine in the fine-tuning. What they determine is your correction: the lens strength with which you see most sharply and most comfortably. That is a craft and an optical achievement, and for many people it is exactly what they need.

The difference is not the care taken but the question. The ophthalmologist examines the eye as an organ: cornea, lens, intraocular pressure, optic nerve head, retina. The ophthalmologist can identify a disease, treat it, prescribe medicines and operate. A glasses prescription says nothing about whether the optic nerve is healthy — and a healthy optic nerve says nothing about whether the glasses fit. They are two different questions put to the same eye.

As a practical rule: If only your prescription has changed and you have no other symptoms, going to the optician is perfectly fine. If your vision has changed and you do not know why — or if you have diabetes, severe short-sightedness, glaucoma in the family, or you see distortions, double vision or shadows —, then the measurement is not the question. The cause is.

What happens, and how long it takes

An eye test with us almost always runs in the same order. You do not have to prepare for it — except to have your glasses with you.

1. Registration and conversation. What is your question? Since when? What gets in the way day to day — road signs, the screen, the book in the evening?
2. Your existing glasses are measured. That takes a minute and is the reference value for everything that follows.
3. Measurement by machine. You look at an image and the device measures on its own. Nothing you can get right or wrong.
4. Fine-tuning with lenses. Now you are involved: 'better like this, or like this?' — until the value is stable, for each eye and then for both together.
5. Near vision. With a reading chart at reading distance, and where needed at the distance of your screen as well.
6. Ophthalmic examination. At the slit lamp, depending on the question with intraocular pressure measurement and assessment of the optic nerve and retina. For the retina, dilating drops are usually needed.
7. Discussion. Findings, prescription, open questions — and a clear proposal as to whether and when a follow-up makes sense.

How long it takes: A measurement of visual acuity on its own is done in just under half an hour. If the ophthalmic examination with a dilated pupil is added, expect forty-five to ninety minutes: the drops need around twenty to thirty minutes to take effect, and you spend that waiting time with us.

Objective and subjective refraction — what that means

'Refraction' is the technical word for determining your correction. It happens in two steps, and the one cannot replace the other.

Objective refraction — what the machine measures

An autorefractometer sends light into the eye and measures how it is thrown back from the retina. From that it calculates how strongly and in which direction your eye refracts the light. You have nothing to answer here — it is a measurement, not a test. That is precisely its value: it works with small children, with tired people and with everyone who is unsure whether they answered 'correctly'. The result is a very good starting point — but it is not your glasses prescription.

Subjective refraction — what you say to that

In the second step we place lenses in front of your eye and ask: better like this, or like this? It is not about right or wrong but about your impression — you cannot get anything wrong here, and 'the two are the same' is a perfectly usable answer. What is fine-tuned is the strength, and the cylinder and axis where there is astigmatism, then how the two eyes work together.

The reason for this second step: seeing happens in the brain, not on the retina. Two people with identical measurements experience the same glasses differently, and the sharpest correction is not automatically the most comfortable. In the end the prescription is a compromise between maximum sharpness and relaxed vision across the whole day — and that compromise can only be found together with you.

With children and young adults a third step is added: their lens can focus so strongly that it temporarily compensates for long-sightedness and so hides it. Drops are then needed that switch this focusing off for a time. Only after that does the measurement show the actual value. These drops wear off markedly more slowly in children than in adults — reading and screens can stay blurred until the next day and the eyes are sensitive to light. Plan the rest of the day accordingly and give the child a pair of sunglasses to take along.

When an eye test is not enough

This is the most important section on this page, even though it is the most uncomfortable. A normal eye test is good news about exactly one thing: your visual acuity, on that day, under test conditions. It is not a certificate of health for your eyes. Two of the commonest conditions that permanently impair vision in later life do not show up on the eye chart for a long time at all.

Glaucoma begins outside the centre of vision — usually in an arc around the centre — and not where the eye chart tests. Central visual acuity — that is, what the eye chart measures — often stays unremarkable for years while the optic nerve is already losing fibres. On top of that, the brain fills missing areas in from the surroundings, so that gaps in the visual field go unnoticed for a remarkably long time. That is recognisable not from the letters but from the measurement of intraocular pressure, from the assessment of the optic nerve head and, where necessary, from the visual field and a cross-sectional scan of the optic nerve. A normal intraocular pressure does not rule out glaucoma — some of these conditions run their course at normal pressure readings. What decides it is the assessment of the optic nerve, not a single number. Lost nerve fibres do not come back — which is why early detection is the whole point here.

Age-related macular degeneration (AMD) affects the point of sharpest vision. Early changes can be present while you are still reading the chart without difficulty — and with both eyes open, the better eye compensates for the weaker one. What people affected often notice first is not blurring but that straight lines appear wavy: window frames, tile joints, lines of text. It becomes visible on the retina itself and in the cross-sectional scan of the macula. Newly distorted or wavy lines — particularly if you notice them when you cover one eye — are not a case for the next free appointment but for a call within a few days.

The same gap applies to other situations:

• Diabetes. Changes to the retinal vessels develop without pain and without any deterioration in vision. Regular monitoring of the retina therefore follows the disease, not how well you feel you see.
• Retina and vitreous. Flashes of light, a sudden swarm of black dots or a shadow coming in from the side are not a question for the next free appointment but for a call the same day — they can be signs of a Retinal detachment .
• Dry eyes. An unstable tear film makes visual acuity fluctuate over the course of the day. Someone who changes glasses three times and is never satisfied sometimes does not have the wrong lenses but a disturbed ocular surface.
• Cataract. The clouding of the lens is troublesome first with glare and at dusk — while the value on the chart can go on looking respectable for a long time.

What follows from that is not drama but a matter of placing it correctly: an eye test answers the question 'which glasses?'. The question 'is everything all right with my eyes?' it does not answer. What a complete ophthalmic examination additionally covers and when it is indicated, you can read on the overview page — and if you are unsure which of the two you need, say so when you book. We settle that question on the phone in two minutes. If what you want is specifically a check when you have no symptoms, the screening examination is the right appointment: eye screening — the assessment of intraocular pressure, the optic nerve and the retina, with its own age bands and its own rule on costs.