A:Answer The 'S' and 'R' stand for Sensitiviy and Resolution, respectively. The a7sii has 12 uniquely large MP to capture tons of light, giving it a professional video advantage. The a7rii has 42MP (in addition to a better processor if I'm not mistaken) making it ideal for incredibly detailed shots. The a7ii has a 24MP, making it the least expensive, almost the base of the a7ii lineup. They do not all shoot 4k video, but all have full frame sensors to produce professional level photos.