In optics,<unk>  is a property of<unk>  systems, such as lenses, that causes light to be spread out over some region of space rather than focused to a point.<unk> s cause the image formed by a lens to be blurred or distorted, with the nature of the distortion depending on the type of<unk> .<unk>  can be defined as a departure of the performance of an<unk>  system from the predictions of paraxial optics. In an imaging system, it occurs when light from one point of an object does not converge into (or does not diverge from) a single point after transmission through the system.<unk> s occur because the simple paraxial theory is not a completely accurate model of the effect of an<unk>  system on light, rather than due to flaws in the<unk>  elements. An image-forming<unk>  system with<unk>  will produce an image which is not sharp. Makers of<unk>  instruments need to correct<unk>  systems to compensate for<unk> .<unk>  can be analyzed with the techniques of geometrical optics. The articles on reflection, refraction and caustics discuss the general features of reflected and refracted rays.
Optical aberration
Defocus aberration
Tissue stress
Vascular anomaly
Environmental factor
Electrical injury
Refractive error
Cell damage