What are barrel distortion, pillow distortion and vignetting of photos?

To put it simply, some wide-angle lenses or zoom lenses will have barrel-shaped distortion with four corners contracting inward, and some telephoto lenses or zoom lenses will have pillow-shaped distortion with four corners extending outward.

The two variants are usually used as a reference when testing or buying lenses, but they are only suitable for testing SLR lenses.

Wikipedia says: vignetting usually occurs due to camera settings and lens restrictions, which is considered undesirable and unintentional. But sometimes it is deliberately added because of the need of creative style, such as attracting attention to the central area of the image. Photographers will also deliberately choose lenses that can produce vignetting to create special effects, or they can use special filters or post-processing to create vignetting.