Diplopia is most commonly known as double vision. It causes a person to see two objects instead of one. The reasons that cause Diplopia are of two major types, i.e. physical change in the lens, and inability of brain to overlay the images seen with both eyes.

The first type, i.e. physical change in lens is mostly associated with one eye only, but it cannot be corrected by covering the eye. This type includes scars or other physical defects in the eye due to which a single image splits into two, and thus results in double vision.

On the other hand, the second type effects both eyes. This type can be corrected by covering one eye. Binocular diplopia [the second type] arises when the eye movement in one direction is prevented, and is often a congenital (present at birth) condition. Binocular diplopia is usually caused by misalignment of the eyes, which can be nerve or muscle related.