Ideally, the curvature of the cornea is shaped regularly. However, when this curvature is more irregularly shaped, this is known as astigmatism. Due to the different curvatures of the cornea, light is refracted differently in, for example, the horizontal plane than in the vertical plane, which means that objects are represented in a blurred and distorted manner.
The interactive animation to the side of this text allows you to see how the eye and image perception change with varying degrees of astigmatism.
The curvature can be optically adjusted so that all rays of light come together in a single point of focus. This usually occurs surgically by means of laser correction or intraocular lenses, but can also occur through glasses with cylinder lenses that need to be positioned in front of the eye in the precisely defined direction. The corrective value of glasses or contact lenses is expressed in dioptres for the cylinder value and in angular degrees for the position of the cylinder axis.