If your eyes do not focus properly due to myopia (nearsightedness), hyperopia (farsightedness), or astigmatism, the optometrist can detect and quantify them using various instruments. The autorefractor is a device that automatically determines the correction your eyes need. However, this only provides a starting point; the optometrist must still fine tune the correction so that the best possible prescriptive lenses can be made.
This is done using a phoropter. You might recognize this as the instrument that is placed in front of your eyes that resembles binoculars. The eye care practitioner will usually start this part of the exam using a retinoscope. This is a handheld instrument that shines a light through the phoropter into your eye. By viewing the reflection from the back of the eye (similar to the red reflex often seen in photos taken with a flash) and changing the lenses in the phoropter, a good approximation of the spectal correction can be determined.
This is all done without any answers from the patient and is used to determine the glasses required by infants, small children, and others who cannot answer questions.
After the eye care practitioner has obtained a good approximation of the correction needed for your eyes, you will be asked a series of questions—"Which is better: one or two?"—while the doctor changes the phoropter lenses to refine the required correction. The ability to focus up close (accommodation) and the correction for reading are also tested. These tests are required as improper accommodation can cause eyestrain and headaches, and are necessary for determining the bifocal power required in spectacles for those over 40 years old who have significantly lost the ability to focus at near distances.