A refraction test, also called a vision test, is usually performed as a part of a routine eye examination. This test aims to determine if a person has a refractive error, which would then mean the patient would need glasses or contact lenses.
A value of 20/20 is normal (optimum) vision. This means that individuals with 20/20 vision can read letters 3/8-inch (1 centimeter) tall from 20 feet (6 meters) away. The normal uncorrected vision (without glasses or contact lenses) refractive error is zero (plano). Individuals who don’t have 20/20 vision have what is called a refractive error. A refractive error means that the light is not bending properly when it passes through the lens of the eye. The refraction test will tell the doctor what prescription lens should be used to achieve 20/20 vision.
For people over age 40 with normal distance vision but difficulty with near vision, a refraction test with a small size is used to determine normal near vision and the correct power of reading glasses.
The test is performed by having the patient seated in a chair with a special device (called a phoropter or refractor) attached to it. The patient looks through the device and focuses on an eye chart 20 feet (6 meters) away. The device contains lenses of different strengths that can be moved into the patient’s view. The test is performed on one eye at a time. If the patient is wearing contact lenses, the lenses should be removed before the test.
If the final vision is less than 20/20 even with lenses, then there is probably another non-optical problem with the eye. The vision level achieved during the refraction test is called the best-corrected visual acuity (BCVA).
Abnormal results may be due to:
Astigmatism (abnormally curved cornea causing blurred vision)
Presbyopia (inability to focus on near objects that develop with age)
Other conditions under which the test may be performed:
Corneal ulcers and infections
Loss of sharp vision due to macular degeneration
Retinal detachment (separation of the retina, the light-sensitive membrane in the back of the eye, from its supporting layers)
Retinal vessel occlusion (blockage in a small artery that carries blood to the retina)
Retinitis pigmentosa (an inherited disorder of the retina)
There is an art to refraction, and the optometrist will always answer the patient’s questions and discuss their findings. Based on the results of the refraction test, they can determine the amount of myopia, hyperopia, or astigmatism.
Children should have a refraction test every 1-2 years, starting at no later than 3 years of age. Healthy adults under age 60 who aren’t experiencing vision problems should have a refraction test every 2 years, while adults currently wearing prescription glasses or contact lenses or with a refractive error should have a refraction test every 1-2 years, or when their vision changes, which may necessitate a change in prescription. If any vision problems arise between exams, the eye doctor should be seen for another refraction test.
It is vital that patients with diabetes have an eye examination every year. A number of eye conditions are associated with diabetes, such as diabetic retinopathy and glaucoma. According to the American Diabetes Association, people with diabetes are at a greater risk for blindness than other Americans.
Adults over 60 or with a family history of glaucoma should also have a refraction test every year. Glaucoma occurs when pressure builds up in the eye, damaging the retina and the optic nerve. Regular exams will help the eye doctor screen for glaucoma and other eye conditions associated with aging and, when necessary, begin early treatment.