Vergence is the simultaneous movement of both eyes in opposite directions in order for a person to maintain visual perception of the object. Basically, this means that the eyes will move in the direction of the object, locking the center of the pupils onto it. This keeps the object in focus at all times, allowing the person to maintain visual contact with it. There are actually two different forms of vergence, one that explains how the eyes work for close objects, and the other for distances farther away.
The form of vergence that deals with objects close, such as looking at the nose or a small sliver in the hand, is convergence. This, in extreme situations, is what is referred to as cross eyed vision. For instance, if a small wood sliver is in the hand, a close up view is needed in order to remove it. The closer that they hand gets to the face, the farther in that the eyes will move, until they are both at a point closest inward towards the nose. In order to see convergence in action, stand in front of a mirror, leaning forward as far as possible, and focus on the center of the nose. The eyes will rotate inward as far as they possibly can so that the centers of the pupils can focus on the point required.
Divergence is the second form of vision, which deals with objects that are farther away. An example of this type of vergence would be a bird flying from left to right in front of a person. In order for the eyes to maintain visual contact with the bird, the eyes will move in the same direction. However, in this instance, the right eye would move outwards away from the nose, while the left eye would rotate towards it. In this aspect, the eyes will remain parallel with each other in order to keep the bird in focus. A good experiment to see divergence in action would be to stand back from a mirror and focus the eyes on a spot to the left or right of it. The eyes will rotate in equal amounts to maintain contact with the object.
Vergence is basically a form of vision which people use in order to maintain focus on any specific object or area. When a problem occurs with convergence or divergence, corrective glasses may be used to help with focus issues. If that does not work, then corrective surgery may be the only option. This form of vision is imperative to have, and when problems arise with it various different strategies will be discussed in order to make sure that vergence, whether close or far, is a vision task that can be easily accomplished.