There are three major causes of amblyopia or 'lazy eye' - squint (also known as strabismus - where the eye muscles are not working properly and one eye is at an angle or turned), very short or long sight in one eye only, and visual deprivation in one eye due to a disorder such as cataract. If the problem is not discovered and treated amblyopia may result. The brain is not able to make the images from both eyes combine into one 3D view of the world, so it ignores the worse image, to avoid double vision. The result is that the patient loses the ability to see in 3D.