Vignetting, less light reaches edges of image than at its center. In EOS cameras with DIGIC 4 processors, Peripheral Illumination Correction evens brightness across image field, automatically reducing light fall-off at image edges for specific Canon lenses.
The size of the area on a surface where light and shadows merge. Soft sources produce a gradual transition, a subtle gradation of tones. Hard sources, without Fill Light, change abruptly. See: Wrap and Contrast.
see Inverse Square Law.