Vignetting – also known as ‘Peripheral Illumination’ or ‘Light Fall-Off’- is the occurrence of underexposed or darkened corners in an image. It can be both an unwanted optical traitor a deliberate choice when shooting.
It occurs when light entering a lens is partially blocked by the lens barrel or when using a lens hood. With modern Canon EOS cameras, the phenomenon can be corrected in-camera, completely seamlessly. On the other hand, there are some photographic situations where a vignette will help to draw attention to the subject, and will be an entirely deliberate choice, for example with Portrait photography. This is why you can actually add in vignetting in post production, for example, with Adobe Photoshop.