What happens at an eye exam?

An eye exam is designed to assess the general state of your eyes and to make sure that they work together in a coordinated way and that the eye muscles are balanced. It can also indicate other health problems — high blood pressure or raised cholesterol, for example.

The optometrist will test your eyesight for both distant and near vision, and establish the correct strength of any lenses you may need.

At the end of the eye exam you will be given either a prescription for glasses (or contact lenses) or a confirmation that your eyesight is fine. An instrument called an ophthalmoscope is used to examine the eye. It is like a long thin flashlight, with a bright light that shines through the pupil, and a magnifying lens to examine the back of the eye. It is used to test pupil and red reflexes.

• Normal pupils constrict or narrow under bright light; the light from the ophthalmoscope will trigger both the direct pupil reflex — narrowing on the same side as the light — and the consensual reflex, in which shining a light in one eye causes the pupil in the other eye to constrict as well. Slow, uneven or abnormal pupil responses can indicate a problem with the nerve supply to the eyes.

• The red reflex utilizes the effect that causes "red eye" in photographs. When the optometrist looks directly at the pupil through the ophthalmoscope, the light hits the back of the eye and is reflected back in the same line. Normally it looks red, owing to the colour of blood vessels in the retina. If this doesn't happen, then a problem with the cornea, lens or retina might be interfering with the light bouncing back.

An ophthalmoscope is also used to rule out eye conditions such as macular degeneration, retinal tears or detachments, as well as to monitor changes that could indicate conditions such as diabetes or high cholesterol.

• If you're over 35 years old, your eye exam will probably screen for glaucoma, which produces changes in the retina that can be seen using the ophthalmoscope.
•  As part of this exam, you will undergo a tonometry test to measure the pressure inside each of your eyes, since high pressure is frequently the cause of glaucoma.
• For this, you will be asked to look through an eyepiece while a special instrument directs a short puff of air onto the front of each eye.
• This is a basic screening test and the results are not conclusive, so your optometrist or ophthalmologist may recommend more elaborate tests.

Sharpness of vision or visual acuity (VA), is also examined. 

• The aim of this part of the eye exam is to find out how well you can distinguish fine detail.
• The optometrist tests each eye separately, while shielding the other, and checks your vision with and without correcting lenses.
• The normal test for this is the familiar chart — called the Snellen chart — which shows rows of letters decreasing in size.
• Your eyesight is graded according to the smallest letters you can read from a fixed distance.
• Different charts are used for very young children or for people who cannot recognize letters. One has simple pictures instead of letters; another, the Tumbling E, shows a series of capital E's, facing in different directions.
• The person being tested has simply to indicate the direction in which the letter is facing.