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Refraction

What does refraction mean?

Refraction is the term used when determining which prescription to provide an individual.

How do you measure vision?

The most common method of measuring vision in adults is the Snellen Test Chart - rows of increasingly smaller letters. Often you will be asked to read the chart using only one eye at a time. An "occluder" or patch is used to cover one eye and you will be asked to read the chart with the uncovered eye and then vice versa. Also, the Rosenbaum "near card" is used to measure vision up close.

How do you measure refractive errors?

Ophthalmologists use various tools to measure refractive error. The automated refractor is a machine used to provide an objective estimate of refractive error. Basically, the patient sits down with their chin on a "chin rest" and they are told to focus on an image inside the machine. The doctor then presses a button to get an estimate of the objective refractive error.

Another type of tool that is used for objective testing is called the retinoscope. Retinoscopy is performed by shining a light into the patient's eyes and by observing the reflex of light off the retina. An estimate of refractive error can then be assessed.

While the automated refractor and the retinoscope provide an objective estimate of refractive error, the phoropter will further refine the refraction and provide a subjective measure of visual acuity. The phoropter is a machine in which patients look through lenses while reading a visual acuity chart. The doctor positions different lenses in front of the patient's eyes until the patient states that they see the clearest. In other words, the patient repeatedly answers questions about which lens gives the best vision. For this reason, the phoropter is not often used on young children. All of the above mentioned tests are painless.

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