One of the most commonly asked questions in an eye exam comes right after the refraction, or glasses prescription check: “What is my vision?”

Almost invariably, people know the term “20/20”. In fact, it’s a measure of pride for many people. “My doctor says I have 20/20 vision.” Or, on the other side of that same coin, having vision that is less than 20/20, say 20/400, can be a cause of great concern and anxiety. In this discussion I will describe what these terms actually mean.

To lay the foundation, let’s discuss some common terms. Visual acuity (VA) is clarity or sharpness of vision. Vision can be measured both corrected (with glasses or contact lenses) and uncorrected (without glasses or contact lenses) during the course of an eye exam. The result of an eye exam boils down to two different but related sets of numbers: your VA and your actual glasses prescription.

The notation that doctors use to measure VA is based off of a 20-foot distance. This is where the first 20...

Here are 11 bad contact lens habits we eye doctors often see--

#1 Sleeping in your contacts.

This is the No. 1 risk factor for corneal ulcers, which can lead to severe vision loss and the need for a corneal transplant. Your cornea needs oxygen from the atmosphere because it has no blood vessels. The cornea is already somewhat deprived of oxygen when you have your eyes closed all night, and adding a contact on top of that stresses the cornea out from lack of oxygen. You don’t need to see when you are sleeping. Take your contacts out!!! I promise your dreams will still look the same.

#2 Swimming in your contacts.

Salt, fresh, or pool water all have their individual issues with either bacteria or chemicals that can leach into your contacts. If you absolutely need to wear them to be safe in the water, then take them out as soon as you are done and clean and disinfect them.

#3 Using tap water to clean contacts.

Tap water is not sterile. See No. 2.

#4 Using your...