In normally-sighted eyes, the optical system deflects the incident rays of light (images) such that they meet precisely at a focus on the retina (usually the point of sharpest vision). This deflection is referred to as the refractive index. The various parts of the optical system of the eye combine to give a total refractive power, which is given in dioptres (D). If this refractive power is correct, the images are portrayed sharply on the retina.
However, if the focus does not lie precisely on the retina but in front of or behind it, we see our surroundings fuzzily. In these cases, we refer to visual defects.