There is a correlation between strabismus and binocular diplopia; strabismus occurs when the optic axes of both eyes are not parallel; strabismus can cause diplopia to occur, such as acute common internal strabismus, intermittent external strabismus and paralytic strabismus, but not all strabismus is diplopia; most people with strabismus do not feel diplopia. Diplopia in the true sense of the word is when both eyes see in pairs. In addition to being caused by ophthalmic disease, binocular diplopia can also be caused by central or systemic disease. Monocular diplopia can be caused by other ophthalmic conditions such as cataracts and lens hemianopia.