Final answer:
Vignetting is the phenomenon that causes image brightness to be greater at the center of an image compared to the periphery, often due to the physical properties of the lens and elements within the optical path.
Step-by-step explanation:
The cause of image brightness being greater at the center of an image compared to the periphery is often due to the phenomenon known as vignetting. Vignetting is a reduction in image brightness or saturation at the periphery compared to the image center. This can arise from various factors, including the physical properties of the lens and the positioning of elements within the optical path, such as the aperture, lens hoods, or filters. It is not a result of lag, pincushion distortion, or quantum mottle—these terms describe other phenomena. To avoid vignetting and other issues like chromatic aberration and spherical aberration, optical systems must find a balance between component quality and computational techniques for correcting aberrations.