Eye Examinations Explained: What Happens and Why It Matters

Many people think of an eye test as reading letters off a chart and walking out with a new prescription. A full eye examination is much more than that. In around half an hour, a trained professional can check how sharply you see, measure the pressure inside your eye, look at the delicate nerve and blood vessels at the back of the eye, and spot early signs of conditions such as glaucoma, diabetic retinopathy and macular degeneration, often years before you would notice anything yourself.

This guide walks you through who does what in eye care, what each part of a comprehensive exam measures and why, how to prepare, and how often different age groups should be examined. Knowing what to expect makes the visit less stressful and helps you ask better questions.

Who does what: ophthalmologist, optometrist and optician

The names sound similar, but the three professions have very different training and roles. Exact titles and legal scope vary from country to country, so always check what your local professional is licensed to do.

For a routine check of vision and eye health, an optometrist is usually the first port of call in many countries. If something abnormal is found, or if you already have an eye disease, you will be referred to an ophthalmologist. For sudden symptoms such as loss of vision or a painful red eye, go directly to an eye emergency service; see our guide to eye emergencies and first aid.

Before the exam: history and preparation

A good exam starts with conversation. You will be asked about your vision, any symptoms (blurring, headaches, double vision, flashes, floaters, dryness, redness), your general health, the medicines you take and eye conditions in your family. Diabetes, high blood pressure, thyroid disease and long-term steroid use are especially relevant, as is a family history of glaucoma or macular degeneration.

What to bring

• Your current glasses (including reading or computer glasses) and, if you wear them, your contact lenses and their box or prescription.
• A list of all medicines and supplements, including eye drops.
• Sunglasses for the trip home if your pupils are going to be dilated.
• Notes about specific problems: when they started, whether they affect one or both eyes and what makes them better or worse.

If you wear contact lenses, ask in advance whether you should remove them before the visit; some tests are more accurate if lenses have been out for a while.

The exam step by step

The order varies between clinics, and not every test is needed at every visit, but a comprehensive examination usually includes the following steps.

Step 1: Visual acuity

Visual acuity is the sharpness of your central vision. You read letters or symbols from a chart at a set distance, one eye at a time, first without and then with your current correction. The classic Snellen chart has given us the familiar term 20/20 (6/6 in metric countries): it means you can read at 20 feet (6 metres) what a person with standard vision reads at that distance. Many clinics now use LogMAR charts, which have the same number of letters on each line and give more precise measurements.

Near vision is tested separately with a reading card, which is particularly important after the age of about 40, when presbyopia begins. If you are curious, you can try a rough home screening with our online visual acuity test, but remember that a screen test never replaces a professional exam.

Step 2: Refraction

Refraction determines the lens power that gives you the clearest vision, and therefore whether you have refractive errors such as myopia, hyperopia or astigmatism.

• Objective refraction: an automated machine (autorefractor) or a handheld light (retinoscope) estimates your prescription without you having to answer questions.
• Subjective refraction: the examiner fine-tunes the result by showing you lenses through a phoropter or trial frame and asking which looks clearer, option one or option two.

There are no wrong answers; if two options look the same, simply say so. In children and some adults, eye drops that temporarily relax the focusing muscle (cycloplegic refraction) are used, because young eyes can focus so strongly that they hide farsightedness. The result is written as a prescription, which our guide on how to read your glasses prescription explains line by line.

Step 3: Eye alignment, movements and pupils

The examiner watches how your eyes move as you follow a target, checks whether they stay aligned when one is covered (the cover test), and tests focusing and convergence at near distance. This can reveal a squint (strabismus), lazy eye or convergence problems that cause eye strain when reading. Pupils are checked with a light: an abnormal pupil reaction can point to optic nerve or neurological problems. Colour vision and depth perception may also be tested, especially in children and for certain jobs.

Step 4: Slit lamp examination

The slit lamp (biomicroscope) is a microscope with a bright, adjustable beam of light. You rest your chin and forehead on a support while the examiner looks at the eyelids, lashes, conjunctiva, cornea, iris and lens in high magnification. It is the main tool for detecting:

• Blepharitis, styes and eyelid problems
• Dry eye, often assessed with a harmless yellow-orange dye called fluorescein
• Corneal scratches, ulcers and infections
• Cataract (clouding of the lens)
• Inflammation inside the eye (uveitis)
• A narrow drainage angle, which raises the risk of angle-closure glaucoma

With an extra handheld lens, the same instrument can also be used to view the retina and optic nerve.

Step 5: Tonometry (eye pressure)

Tonometry measures intraocular pressure (IOP), the fluid pressure inside the eye. Raised pressure is the main modifiable risk factor for glaucoma. Normal pressure is usually quoted as roughly 10 to 21 mmHg, but a reading outside this range does not by itself mean you have, or do not have, glaucoma.

• Non-contact (air-puff) tonometry: a brief puff of air flattens the cornea. It is quick and used for screening.
• Goldmann applanation tonometry: after anaesthetic drops and a little fluorescein, a small probe gently touches the cornea. This is the reference standard.
• Rebound and handheld tonometers: useful for children and people who cannot use a slit lamp.

Corneal thickness (pachymetry) may also be measured, because a thick or thin cornea can make pressure readings look higher or lower than they really are.

Step 6: Dilated fundus examination

To see the back of the eye properly, the examiner often uses drops that widen (dilate) the pupil. These take about 15 to 30 minutes to work. Through the enlarged pupil the retina, macula, optic nerve and blood vessels can be inspected in detail, including the far edges of the retina where tears and detachments begin.

A dilated exam is important for detecting diabetic retinopathy, macular degeneration, retinal tears, glaucoma damage to the optic nerve and, occasionally, tumours. The drops may sting briefly. Afterwards, close-up vision is blurred and bright light feels uncomfortable for several hours. Do not drive until your vision has returned to normal, and wear sunglasses outdoors.

Step 7: Optical coherence tomography (OCT)

OCT is a fast, painless scan that uses light waves to create cross-sectional images of the retina and optic nerve, rather like an ultrasound but with far finer detail. You look at a target while the machine scans for a few seconds; nothing touches your eye.

OCT can show swelling of the macula in diabetes, fluid under the retina in wet macular degeneration, macular holes and the thickness of the nerve fibre layer, which thins in glaucoma. It is also used to monitor how well treatments such as anti-VEGF injections are working. Retinal photographs are often taken at the same time to document the appearance of the eye for future comparison.

Step 8: Visual field test

A visual field test (perimetry) maps your side vision. You look at a central light inside a bowl-shaped machine and press a button each time you see a faint flash elsewhere. Each eye is tested separately and the test takes a few minutes per eye.

Visual fields are essential for diagnosing and monitoring glaucoma, which usually damages side vision first, and for detecting problems along the visual pathway such as optic nerve disease, strokes or pituitary tumours. It is normal to miss some flashes; the machine accounts for this. Results improve with practice, so your first test may be repeated.

Other tests you may be offered

• Gonioscopy: a special contact lens to inspect the drainage angle in glaucoma assessments.
• Corneal topography: a map of the corneal surface, used for contact lens fitting, keratoconus and before laser surgery.
• Biometry: measurement of eye length before cataract surgery to calculate the lens implant power.
• Amsler grid: a simple grid to detect distortion from macular problems; you can practise with our Amsler grid test on this site.
• Fluorescein angiography: a dye injected into the arm to photograph retinal blood flow.

How often should you have an eye exam?

Recommendations differ slightly between organisations and countries, but the general guidance from bodies such as the American Academy of Ophthalmology, the American Optometric Association and the NHS is broadly similar. Your own examiner may advise a different interval based on your results.

For more detail on intervals, read how often you should get an eye exam, and our children section for advice on younger patients.

Screening versus a full exam

Vision screenings at school, at work or for a driving licence check only basic acuity. Passing one does not mean your eyes are healthy. Likewise, a quick test at an optical shop may focus mainly on your prescription. If you have risk factors or symptoms, ask specifically for a comprehensive examination including eye pressure and a view of the retina.

Can eye exercises replace an exam?

No. Eye exercises can help with comfort and relaxation, and a few specific exercises are used under professional guidance for problems such as convergence insufficiency. They cannot correct refractive errors or detect, prevent or cure glaucoma, cataract or retinal disease. Regular exams remain the only reliable way to catch these early. Our overview of the evidence on eye exercises explains what they can and cannot do.

When to see a doctor urgently