There are several menu settings that affect when the red illumination of focus points is activated.

1. By default, the red indicator lights will only illuminate over the active focus points when focus is achieved and the brightness of the scene in the viewfinder is low. You can modify this in the menu settings. Under the AF 5 (purple) tab, select VF display illumination. The Auto option is the default setting. Select Enable to have the points light up regardless of the brightness of the scene. Select Disable if you do not want them to light up. It is explained on page 103 of the 5DIII user manual.

2. The red lights will only blink over whatever black indicators are visible. This may be all, some, or none of the focus points depending on the setting chosen under AF point display during focus. To change which focus points are displayed and when they are displayed in the viewfinder, see page 102. This adjusts the black non-illuminated display of the focus points.

3. To turn on the red illumination when focus has not been achieved, Press the AF point selection button. This is useful for checking and changing the AF focus points/zones when shooting in a dark environment. What is illuminated will depend on the AF Area Selection Mode. See pages 72-77 for how to select an AF Area Selection Mode and how to manually select AF points or AF zones.

Focus confirmation is also communicated by the beep (if it is turned on under the Shooting 1 (Red) menu) and the green focus confirmation light in the lower right of the viewfinder. See page 23 of the 5DIII user manual.