Posted on September 09 2015
There are a number of elements that can impair our sight. In order to properly understand these elements, though, it is best to start by discussing a completely functioning eye. Sometimes, people use the phrase “perfect vision,” but usually eye-care professionals discuss “normal vision.” In other words, normal vision is what someone should be able to see without assistance.
Emmetropia is the term for vision that does not need any correction. In this condition, light waves hit the retina at exactly the proper position to be registered by the photoreceptors (rods and cones) in the retina. With emmetropia, the eye needs to exert no effort to see distance clearly. Our eyes will naturally see distance more easily than closer objects. When viewing objects at close range, there is always movement inside the eye, even for someone with normal vision. The crystalline lens inside the eye needs to bend in order to focus the light waves of close objects onto the retina. However, when we view something at a distance (usually classified as 20 feet away), the crystalline lens is in its relaxed state, flat and straight. If someone can see an object 20 feet away clearly without concentrated effort, they have normal, or emmetropic, vision. If someone needs to squint, which is moving the eye lens to focus light on the retina, they do not have normal vision.
How exactly does an emmetropic eye see clearly? Light waves from an object we are viewing at a distance will reach our eyes along parallel lines, at the top and bottom of the cornea. The curve of the cornea bends, or refracts, those parallel lines slightly towards each other. That is because the cornea is a convex shape, which causes light waves to converge. The light waves are then refracted again by the crystalline lens, another convex shape, causing the light waves to converge towards each other even more. Those refracted light waves will eventually meet at a certain point. If the length of the eye is proper, the point where those converging light waves meet will be directly on the retina.
The way that eye-care professionals have traditionally determined emmetropia is through a vision test. Using a Snellen chart, which features letters in rows of different sizes, an eye-care professional determines whether we can focus on letters at a distance of 20 feet with a relaxed eye. If someone has 20/20 vision acuity, they can see “normally,” meaning they can see clearly at 20 feet without aid. That distance is what the first 20 refers to (the second 20 relates to the size of the letters). It is possible to have better than 20/20 vision. If you have 20/15 vision, it means that what a normal person can see at 15 feet you can see from 20 feet away. Conversely, if you have 20/40 vision, it means that what a normal person can see at 40 feet, you have to be 20 feet away to see.