You can choose the focusing distance range with a switch. By setting a suitable focusing distance range, actual autofocusing time can be shorter.
¡EF135mm f/2L USM :
q0.9 m / 3.0 ft. – ∞ w 1.6 m / 5.2 ft. – ∞ ¡EF200mm f/2.8L II USM :
q1.5 m / 4.9 ft. – ∞ w 3.5 m / 11.5 ft. – ∞ ¡EF300mm f/4L USM :
q2.5 m / 8.2 ft. – ∞ w 6 m / 19.7 ft. – ∞ ¡EF400mm f/5.6L USM :
q3.5 m / 11.5 ft. – ∞ w 8.5 m / 27.9 ft. – ∞
If you autofocus outside the set focusing distance range, the lens may stop focusing at the start of the focusing range; however, this is not a malfunction. Press the shutter release button halfway again.
Infinity compensation mark
Distance index 
To compensate for shifting of the infinity focus point that results from changes in temperature. The infinity position at normal temperature is the point at which the vertical line of the L mark is aligned with the distance indicator on the distance scale.
For accurate manual focusing on subjects at infinity distance, look through the viewfinder while rotating the focusing ring.