Why the corners receive less light
A camera lens sends light toward a flat sensor from many angles. Light reaching the center travels nearly straight through the lens, while light destined for a corner arrives at a steeper angle. As the angle grows, the lens delivers less illumination there: the sensor receives the light spread over a larger area, and the lens’s effective opening appears smaller from that off-axis path. This gradual edge falloff is called natural vignetting.
A smooth falloff, not necessarily a flaw
Natural vignetting often appears as corners that are a little darker than the center, with no sharp boundary. It is distinct from mechanical vignetting, which occurs when a lens barrel, filter, or other obstruction physically blocks some of the light near the frame’s edges. Closing the aperture can reduce certain kinds of corner darkening, especially when the lens itself is partly obstructing oblique rays, but it cannot eliminate all natural falloff.
Photographers may leave the shading in place when it helps draw attention toward the middle of the frame, or correct it in camera or during editing when even brightness matters. The same effect can be more noticeable with a wide-angle lens or a large aperture. In either case, the darkening is a consequence of how off-center light passes through the optical system—not necessarily a deliberate border added to the image.
