No lens is optically perfect. In the ideal world, every lens would produce a sharp, undistorted, evenly illuminated image that’s sharp from edge to edge. Some lenses come close, but the fact is that lens designers have to work within the limitations of the laws of physics and the glass materials they have available, and […]
An effect where the edges of the picture are darker than the centre (also called corner shading). It was common with old lenses and it's become associated with a vintage look. It's considered a lens aberration these days, though photographers often like to add a vignette effect deliberately.