Originally Posted by ruby.monkey
The rear elements of SLR lenses typically sit further from the sensor than do R/F lenses (made for film cameras anyway), so light hits the edges of the sensor at a less extreme angle and so causes less light fall-off.
Wide-angle R/F lenses can show less distortion that do equivalent SLR lenses so they do have their uses.
The angle is extremely important.
With film the light angle has a trivial impact. The photosensitive particles are essentially randomly distributed in a three-dimesional space. So Gauss designs and retrofocus designs are angle insensitive except for ultra-wide angle lenses where the latter typically has less vignetting.
Digital cameras are very sensitive to light angle. A photosite can only respond to light at a limited range of angles. On-sensor microlenses and other advances minimize this limitation to some degree. Fujinon XF lenses are just one example of many optical designs that attempt to optimize the light angles over the entire sensor surface.