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Your eye test explained

Eye tests play a huge part in making sure your eyes stay healthy. Not only are they important for indicating whether you need to start wearing glasses or need to change your prescription, but also for detecting some common eye conditions.

What happens at an eye test?

An eye test involves a series of checks to assess your vision and check the overall health of your eyes. More than checking your vision, it’s an overall eye health check, inside and out. Eyes can be affected by several conditions which may be picked up early through an eye test, giving it less chance of affecting your vision. Your eyes can even indicate signs of general health problems like hypertension and diabetes.

Diseases found by an eye test can, very rarely, be life-threatening, so eye examinations are an important part of your regular health checks. There are four main parts involved in a typical eye test:

History and symptoms

This gives you the opportunity to discuss any concerns you have about your eyes, vision or current glasses, and any symptoms you’re experiencing which might need further investigation. The optometrist will ask about your general health, medications, any past treatment on your eyes, or any family members with existing eye conditions. This information helps us to tailor the vision test to best suit your needs.

The pre-tests

Before your full eye exam, you will have a pre-test with one of our optical assistants. Depending on your individual needs, a variety of different pieces of diagnostic equipment can be used in the pre-test, including: 

Eye pressure test (tonometer):

A tonometer will blow a gentle puff of air onto the surface of each eye to measure the internal pressure, which can help assess your risk of developing glaucoma.

Digital retinal photography (DRP):

The optical assistant may take a photograph of the back of your eyes using a fundus camera. This image will be saved to monitor any changes in eye health on future visits

Autorefractor test:

This machine measures the ability of your eyes to focus, helping to assess how long- or short-sighted you are. You will stare into the machine through two lenses and focus on a picture appearing closer and then further away, which helps to calculate an estimation of your prescription.

Learn more about autorefractor tests here

The main eye tests

The optometrist will then check the health of your eyes and look for signs of other medical conditions. They will ask you if you are experiencing any eye problems and about your general eye health and lifestyle. It is important to have a clear understanding of your needs so that we can select the best management plan and/or corrective lenses for you.


This vision test is used to assess how well your eyes can focus, helping to determine how long- or short-sighted you are. While focusing on something in the distance, your optometrist will shine a light in each eye, placing different lenses in front of them to help calculate your prescription.

Occasionally, your optometrist may use pupil dilating drops to get a clearer view of the eye.

Find out more about dilated eye exams.

Snellen test:

The optometrist will ask you to read from a Snellen chart (also known as the letter chart test).

This measures your visual acuity (i.e. how well you can see with and without lenses in front of your eyes) to determine whether you have 20/20 vision or whether a prescription is needed to give you the best vision possible.


This specialist torch is used to examine the retina at the back of the eye, your optic nerve and its blood vessels to make sure they are healthy.

Slit lamp test:

A slit lamp (also known as a Volk lens) is a powerful microscope used to examine the front surface of the eyes (cornea, iris and lens) for abnormalities or scratches, which is particularly important for contact lens wearers.

Visual field check:

This test will assess your ability to detect flashes of light in your peripheral (outer) vision. Visual field tests are often used to detect early stages of glaucoma or any conditions that could be associated with headaches and other health issues.

OCT scan:

OCT (Optical Coherence Tomography) is a hospital-grade eye scan that helps to view the structures of your eye in greater detail, helping spot signs of eye conditions like glaucoma and diabetic retinopathy up to four years earlier than traditional methods.

They only take a few seconds to complete — just ask your local store if they offer OCT scanning so you can add it to your normal eye test.

Summary and advice

Once all the relevant tests have been carried out, your optometrist will be able to discuss the results and offer any advice. This could be a simple ‘all clear’ on the eye health front, or they might talk you through particular lens options to match your prescription and lifestyle. Treatments for any eye conditions are also discussed at this point, as well as possible referral to a specialist.

Do I need an eye test?

It’s recommended that you have an eye exam at least every two years, however if you’ve noticed a change in your vision, it’s best to get your vision tested.

Check out our resources below to help you get started.

Online eye test

Check your vision on our online eye test

Book an eye test

Find your nearest store

NHS eye tests

Find out if you’re eligible for a free NHS-funded eye test

DVLA eye tests

What to expect in your DVLA eye exam

Kids eye tests

Find out more about our children’s eye test process

Your contact lens appointment

Find out about contact lens appointments and how they differ from an eye test