Vignetting has a number of causes, but the end result is the same—the image is darker at the edges than in the center. You always get a stronger vignetting effect at the widest aperture of the lens. Stopping down significantly reduces this problem (see Figure 3 and Figure 4). Although vignetting is a technical problem, it's not always a creative one. The effect can enhance some images, such as portraits. Do you prefer Figure 3 or Figure 4?
Figure 3 I used an 85 mm lens and an aperture of f2 to take this portrait.