"Negative dysphotopsia" is the term used to describe the "dark shadows" that a few intraocular lens (IOL) patients see in their far peripheral vision. The primary cause is highly likely to be the limited size of the IOL in comparison to the natural crystalline lens that it replaces. At very large visual angles the light is no longer imaged by the IOL, and a shadowlike phenomena may be visible. This is actually an imaging property related to IOLs, rather than the casting of a shadow.
This phenomenon is now being evaluated systematically in a series of scientific papers. The papers themselves contain references to earlier work. The work so far indicates that this visual phenomenon is an expected characteristic of the use of an intraocular lens. This has also led to additional questions about far peripheral vision.