Concept: The unequal refractive state of both eyes, regardless of the nature or degree of refractive error is called refractive aberration. Clinical manifestations: 1. Asymptomatic in mild cases. 2. Visual fatigue and reduced visual acuity (>2.50D). 3, can produce alternating vision, easy to occur in the eye with good vision in both eyes. 4. Monocular vision, strabismus or amblyopia (eyes with higher degrees) Treatment Less than 3D – glasses correction. Greater than 3D – contact lenses, surgery.