An optometrist has several options to test for astigmatism, or a mis-shaped portion of the cornea. Corneal topography is a method whereby a circular shaped light is shined onto the cornea. The shape of the reflected light is noted. If it is not circular (more oval shaped, for instance), then that is a sign of astigmatism. This can either be determined through visual results or the data can be captured and recorded by computer.
The optometrist can also used phoropter to test for astigmatism. A phoropter is the large device you look through that they use to measure your eyes' corrective prescription (is this better or this better...). Certain pictures will look distorted or unevenly colored if astigmatism is present. Rest assured that a licensed optometrist can effectively correct astigmatism in their patients.
The doctor may use one or more of these four tests to diagnose astigmatism and measure its severity:
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. The doctor uses loose lenses or a lens machine (phoropter) to hold corrective lenses in front of your eye. The doctor looks at the light reflex from your eye through a handheld device called a retinoscope 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 based on which corrective lenses worked for you.
Keratometry. This machine measures the curvature of your central cornea. Corneas that have no astigmatism exhibit uniform or symmetrical curvature, while corneas with astigmatism do not have uniform curvature. The lens can also lack uniform curvature and thus contribute to astigmatism in the eye. 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 three-dimensional picture of your cornea. Such sophisticated measurements are important for planning refractive surgery, cataract surgery, and occasionally for fitting contact lenses.