Amount of light reaching the sensor of pinhole camera model
This effect is called vignetting. The cases you talk about are specifically natural and pixel vignetting as explained on the wiki page. Digital cameras compensate these effects, but optical vignetting has to be done in image software for DSLR's with interchangeable lenses. For example if you import an image to Lightroom, there's "lens correction" option, which removes vignetting, distortion and chromatic aberration caused by the lens based on the lens profile.