I really, really like the Canon extension tubes, I've been using them regularly since 2007 and always carry at least a 12mm one with me. I use them primarily behind the 24-70, I photograph a lot of flowers and like to be close and kind of wide but notice I do a lot in the 50mm range. I've used them with the 5D classic and IDMKIII. Quite frankly I prefer manual focus while using the depth of field preview to move my focus to where it is visually the nicest. I've got 2-12mm and 1- 25mm but I shoot mostly close ups rather than actually macro shots. I've used them with the 24-70, 70-200 2.8, 24-105, 100-400, 85 1.2, 135L, 200L 2.8 and always had great results. Re, auto focus, when I use it, I've never noticed the speed so I guess it isn't an issue. Most of the time I like to be stopped down to at least a 5.6 unless using the 85 1.2 which is really cool to use on flowers wide open.