Vision test. Using a standardized chart, patients read the letters they can see from 20 feet away. If your vision is 20/20, you can see at 20 feet what a normal eye can see from 20 feet. If your vision is 20/80, you can only see at 20 feet what a normal eye should be able to see from 80 feet away.
Refraction. A lens machine (phoropter) that contains corrective lenses is positioned in front of the eye and used to measure your prescription. Sometimes, the doctor looks at the light reflex from your eye through a handheld device called a retinoscope and uses handheld lenses to determine if any refractive error is present. Then the eye doctor will offer you different choices in lenses through the phoropter to refine the correction until you can see the vision chart clearly. After both eyes are measured, your doctor will write a prescription for eyeglasses or contact lenses based on which corrective lenses worked for you.
Keratometry. This machine measures the curvature of the central cornea. The cornea is the outer lens of your eye, like a car windshield. Corneas that have no astigmatism exhibit uniform or symmetrical curvature, while corneas with astigmatism do not have uniform curvature. The keratometer determines the steepest and the flattest curves of the cornea, measurements that tell your doctor about your corneas' shape and focusing power. The keratometer is also used to fit contact lenses and to monitor corneal curvature after eye surgery.
Corneal Topography. This advanced technology provides the most detailed information about the shape of a cornea. The patient looks at a visual target while the device collects thousands of tiny measurements. A computer then constructs a color map on the computer from the data. This corneal map allows the doctor to see a comprehensive picture of your cornea. Such sophisticated measurements are important for planning refractive surgery, cataract surgery, and occasionally for fitting contact lenses. Corneal topography is important in the diagnosis of keratoconus as a cause of astigmatism.