Canon EF 24-70mm f2.8L II USM - Manual (Page 9)

ENG-8
5. Infinity Compensation
Mark
For accurate focusing in MF on subjects at infinity
distance, look through the viewfinder while rotating
the focusing ring.
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 distance
scale L mark is aligned with the distance index.
Distance index
Infinity compensation mark
COP
Y