Regular eye tests are important, not just to check on your sight but also because a routine check will include tonometry to pick up signs of glaucoma and an examination of the optic nerve at the back of the eye to make sure the retina is healthy.
You should have your vision tested once every two years, especially if you are over 40 years old. The most common vision tests assess the sharpness (acuity) of your distance vision and how well your eyes focus on near objects. The tests also show which corrective lenses you may need. An additional test for glaucoma (see Applanation tonometry, below) may be performed, depending on your age and medical history. Phoroptor This device holds different lenses in front of each eye, allowing the optician to test each eye separately.
Snellen chart The sharpness of your distance vision is tested separately for each eye. The test involves reading letters of decreasing sizes on a Snellen chart. Near-vision test Wearing your glasses or contact lenses, if you use them, you will be asked to read very small print on a chart held at normal reading distance. This tests how well you can focus on near objects.
Having a vision test Lenses in the phoroptor are changed until you can read letters near the bottom of the Snellen chart, enabling the optician to make the appropriate prescription for your corrective lenses.
Applanation tonometry The condition glaucoma, in which the pressure inside the eye is raised, can be detected using an instrument called an applanation tonometer. Anaesthetic drops may be put into your eye test distance vision. Each eye can be tested separately.
and the tonometer is then gently pressed against the cornea (the transparent front part of the eye) to measure the force needed to flatten the cornea. The test lasts only a few seconds and is painless.
Vision tests for children
Vision tests in children are designed according to their age and ability and are performed on a regular basis to look for defects that may delay normal development and learning. Vision can be assessed in infants using special tests, while older children can match shapes or letters. Once a child can read, adult vision tests can be used (see Snellen chart, above). Retinoscopy This test may be performed on infants. About 30 minutes before the test,
eye drops are given to dilate the pupils and prevent focusing, and a beam of light is shone into each eye in turn from an instrument called a retinoscope. The effect of different lenses on the beam of light tells whether the child needs glasses.
The test is performed in a darkened room. Letter-matching test This test is usually used for children from about the age of three who are able to recognize letters.
A child is given a card with letters printed on it. The optometrist then holds up cards with letters of decreasing size at a distance of 3m (10ft) and asks the child to identify the same letters on the card he is holding. Each of the eyes can be tested separately using an eye patch.