Aberation of lower rank
This is a defect of vision which can be corrected by glasses or contact lenses.
In order for the parallel rays of light coming into the eye to fall precisely at the sharpest point on the retina, the correct ratio of its length and thickness of the optical system of the eye must be maintained.
If the eye is too long or too short, the focus of the converging rays falls outside the retina. This imbalance manifests itself in the form of three basic refractory errors: short-sightedness, far-sightedness, and astigmatism