A refraction is usually performed as a part of a routine eye examination. The purpose of this test is to measure the refractive error or power that a patient needs in order to see. This power could then be put into glasses or contact lenses to improve a patient's vision.
Visual acuity is a value that is used to grade the clarity of a person's vision. A visual acuity of 20/20 is considered "normal/optimal" vision on a Snellen visual acuity scale. This means that individuals who have 20/20 vision are able to read letters that are approximately 8.75 mm tall from 20 feet away. People with "perfect vision" have an uncorrected (without glasses or contact lenses) refractive error (power) of zero. Individuals who don’t have 20/20 vision, have refractive errors which means that the light is not bending properly when it passes through the cornea and lens of the eye. The refraction tells the doctor what prescription lens should be prescribed in order to have 20/20 vision.
For people over age 45 who have difficulty with near vision, a refraction test with small type size is used to determine near refractive error and the correct power of their reading glasses.
A refraction is performed in various different ways, but the most common method uses a special device called a phoropter. The patient looks through the phoropter and the doctor measures the power of the patient's eyes while they focus on an eye chart that is 20 feet away or optically 20 feet (achieved by using mirrors). The phoropter contains multiple lenses of different strengths and can create hundreds of thousands of combinations of power. The test is performed one eye at a time. If the patient is wearing contact lenses, they should be removed before the test. The vision level achieved during the refraction test is called the best-corrected visual acuity (BCVA).
Astigmatism (Corneal or lens curvature causing direction specific refractive error)
Presbyopia (inability to focus on near objects that develops with age)
Doing an accurate refraction is an art and different methods must be used for different patients to control accommodation or involuntary focusing of the eyes. Dr. Kubota will always explain and discuss the results of your refraction.