An optometrist or ophthalmologist will check your eyes with a few tests during an eye exam:2

• Visual acuity test: This test measures how well you can see and is a routine part of eye exams. An eye care specialist will ask you to read letters on a card or chart 20 feet away. Visual acuity is expressed as a fraction: The top number refers to the distance you stand from the chart, which is often 20 feet, and the bottom number indicates the distance at which a person with normal eyesight could read the same line you correctly read.5 Normal distance visual acuity is 20/20, while 20/70 to 20/160 is considered moderate visual impairment and 20/200 to 20/400 is considered severe visual impairment. Studies have shown that both near and distance visual acuity are affected in astigmatism.6
• Keratometry: This test is specifically used to check for astigmatism. During this test, your eye doctor will check your cornea using a keratometer, which measures the curvature of the cornea by focusing a circle of light on the cornea and measuring its reflection. Keratometry can also determine the amount and axis of astigmatism (where the astigmatism is located on the cornea).
• Corneal topography: This imaging generates a contour map of the cornea and provides even more detail of the cornea's shape. This test can also be used to fit contact lenses.
• Refraction: Your eye doctor will place multiple lenses in front of your eyes using an instrument called the phoropter to measure how they focus light. The specialist will use a retinoscope (a handheld, lighted instrument) or an automated instrument to evaluate the approximate focusing power of the eye. The power is refined to determine the lenses for clearest vision based on your responses.