Canon EOS-1D X Mark III EOS-1DX Mark III Advanced User Guide - Page 908

Compatible Lenses and Autofocusing (Viewfinder Shooting) Group A Group B Group C Group D Group E Group F Group G Group H Group I Group J Group Classification of Lenses Caution Although the camera has 191 AF points, lenses are classified into 10 groups (A- J) which vary in the number of AF points available, AF point patterns, Area AF frame shapes, and other details. When lenses in groups F-J are used, fewer AF points are available. See Group Classification of Lenses for lenses you will use. Note AF points in positions indicated by [ ] blink when the < > button is pressed (while [ / / ] points remain lit). For details on AF point blinking/illumination, see Meaning of Lit or Blinking AF Points. For updates on "Group Classification of Lenses," visit the Canon website or others. Some lenses may not be available in certain countries or regions. 908