Anisometropia is where the eyes have markedly different prescriptions, resulting in different image sizes being presented to the brain. The brain is quite sensitive to this and can only tolerate fractional differences in image size. Some people have healthy eyes but just happen to have different prescriptions in each eye others end up like this after surgery. As contact lenses sit on the eye, the differences in image size are eliminated such that you could be -50 in one eye and +50 in the other eye but provided you wear contact lenses in both eyes, you are unaffected. Contact lenses for children with anisometropia are commonly used to avoid them developing lazy eyes.