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 […]
Corner shading
Corner shading is another term for vignetting, where the edges of the picture are darker than the center. It's because the lens is illuminating the sensor unevenly, and most camera lenses have optical designs that reduce or eliminate this vignetting effect.