In response to user requests, Canon is proud to offer Firmware Update Ver. 1.1.1, which greatly enhances the sophisticated 61-point High Density Reticular AF System of the EOS-1DX DSLR. This firmware update adds two important autofocus capabilities: AF point illumination during AI Servo AF and cross-type AF at f/8.
AF Point Illumination During AI Servo AF 
To support the needs of wedding, portrait, sports, wildlife, and theatrical performance photographers working in low light, AF points in the EOS-1D X's viewfinder can now be illuminated in red with a half-press of the shutter in AI Servo AF mode, for easy viewing of the selected point. Illumination is intermittent, not continuous, and does not interfere with metering accuracy. Two brightness levels are available for the most suitable level of illumination in dark settings and illumination is fully compatible with the range of focus points and focus zone selection options on the EOS-1D X.
Cross-Type AF Available For Maximum Apertures As Small As f/8 
Ideal for wildlife photographers and others who often use Canon EF super-telephoto lenses with extenders, the new firmware allows cross-type autofocusing with the center AF point when the maximum aperture becomes as small as f/8. This new feature greatly expands the range of EF lens/extender combinations that support autofocus when used with the EOS-1D X.
For more information on compatible lenses and suggested menu settings please visit Canon's Website here.