When the eyes can not focus on the same image, amblyopia may happen - this is also called strabismus. Amblyopia often happens in childhood, and it usually affects only one eye.

If an eye is highly nearsighted, amblyopia may occur, because the brain often ignores blurry images. Other factors, such as cataract, premature birth or low birth weight, may also cause amblyopia.

When one eye is weaker, and the other eye is stronger. The brain tend to use the stronger eye, but ignore the weaker eye. This makes the stronger eye to develop good vision, while the weaker eye has poor vision. Early treatment can often reverse amblyopia.