Vignetting is an imaging phenomenon that happens with virtually every optical system. It can even be added intentionally in post-production. Vignetting appears as a radial darkening toward the corners of a frame, and can actually arise for several fundamentally different reasons:
Natural vignetting appears as a gradual darkening and is primarily caused by light reaching different locations on the camera sensor at different angles (see next section for more). This type of vignetting is most significant with wide angle lenses.
Optical vignetting is also gradual, but is primarily caused by intrinsic lens characteristics and shading from the lens barrel itself. It's also what ultimately determines the size of a lens's imaging circle. This type of vignetting is more pronounced at wider apertures, and is strongly influenced by the particular lens design.
Mechanical vignetting typically happens abruptly and only in the corners, since this is caused by matte boxes, filter rings or other objects physically blocking light in front of the lens. This type of vignetting appears less abrupt at wider apertures and with zoom lenses, it can often be eliminated by using a longer focal length.
All types of vignetting appear more pronounced at far focusing distances and with lower crop factors, or when high contrast tonal curves are added in post-production. Mechanical vignetting is the only type that can be fully eliminated by camera technique, but only optical and natural vignetting can be addressed in post-production.
See full details and examples of lens vignetting on the RED 101 Article: https://www.red.com/red-101/lens-vignetting