To remove vignetting from a picture or add vignetting, use Edit | Adjust | Vignetting… [Ctrl+Shift+V] in the Browser or Adjust | Vignetting… in the Editor.
Vignetting is uneven exposure throughout a photo, with darkening towards the corners. Typical causes of vignetting are add-on lenses and filters for wide-angle lenses, and low-quality lenses.
When Automatic mask mode is active, the brightness correction mask is created automatically based on the picture.
When it is off, the mask is created based on the Radius setting, which sets how quickly the picture darkens towards the corner. This second antivignetting mode is only useful for uncropped pictures, because it always places the center of the correction in the center of the picture. Use the Effect strength slider to set the amount of vignetting to add or remove. (To add vignetting, use negative values.)