Horopter

When we focus on an object up close, our eyes adjust so that we are able to see the object in single vision. When we do this, there is an area surrounding the object which we also see in single vision. This area is called the Horopter.

For example, if you were to hold your thumb close a couple of inches in front of your face and you focused your eyes on it, you would be able to see it clearly in single vision. At the same time, there is an area that extends from your thumb, which you also see in single vision. That is the Horopter. Beyond your thumb, or farther away from the Horopter, your vision would be blurry or you would see double images.