We’ve all experienced more than a few eye exams. Few people actually know what the instruments your eye doctor uses are really called and the purpose of many of them is not clear. Here are the three main instruments you’ll likely encounter in a visit to you doctor and what they do.

 Phoropter

The phoropter is the most common instrument. It’s that giant mess of lenses through which you look at an eye chart through while the doctor asks if “one or two” is better. It basically allows the doctor to narrow down the exact refractive index needed to give you near 20/20 vision.

Tonometer

The tonometer is another common instrument; there are two major varieties. One uses air puffs to apply pressure to your eye while the other actually presses on your eye with a rod. The later requires the use of numbing drops to keep you from blinking. Your optometrist is looking to see if there are any irregularities in your eye’s shape and the fluid inside of your eye. The device can allow your doctor to diagnose glaucoma early and help you get the appropriate treatment in time.

Autorefractor

An autorefractor is another boxy instrument for detecting the refractive error of an eye. Like with the phoropter, you look through the device at an image, in this case a picture. However, the autorefractor will automatically move in and out of focus and will examine how your eye reacts to the image. It is mainly used as an additional step after the use of the phoropter to confirm and dial in the ability of the eye to focus.

Going to the eye doctor can be a chore, but knowing how and why the doctor is having you do all those silly things can make the experience more enjoyable and engaging.