Double vision (diplopia) is seeing two images of one object. Double vision may occur when only one eye is open (monocular diplopia) or, more commonly, when both eyes are open (binocular diplopia). Binocular double vision disappears when either eye is closed. Other symptoms, such as eye pain, bulging eye, or muscle weakness, can be present depending on the cause of double vision. Causes of diplopia may include:

Cornea problems.
Problems with the cornea often cause double vision in one eye only. Covering the affected eye makes the double vision go away. The abnormal surface of the eye distorts incoming light, causing double vision. Damage can happen in several ways:
--Infections of the cornea, such as herpes zoster, or shingles, can distort the cornea.
--Corneal scars can alter the cornea, creating unequal visual images.
--Dryness of the cornea can create double vision.

Lens problems
Cataracts are the most common problem with the lens that causes double vision. If cataracts are present in both eyes, images from both eyes will be distorted. Cataracts are often correctable with minor surgery.

Muscle problems
If a muscle in one eye is weak, that eye can't move smoothly with the healthy eye. Gazing in directions controlled by the weak muscle causes double vision. Muscle problems can result from several causes:
--Myasthenia gravis is an autoimmune illness that blocks the stimulation of muscles by nerves inside the head. The earliest signs are often double vision and drooping eyelids, or ptosis.
--Graves' disease is a thyroid condition that affects the muscles of the eyes. Graves' disease commonly causes vertical diplopia. With vertical diplopia, one image is on top of the other.