Infinity Compensation Mark

To compensate for shifting of the infinity focus point that results from changes in temperature, there is a margin at the infinity (∞) position.
The infinity position at normal temperature is the point at which the vertical line of the distance scale  mark is aligned with the distance index.
 

For accurate focusing in MF on subjects at infinity distance, look through the viewfinder or at an enlarged image on the LCD monitor* while turning the focusing ring. * On cameras capable of Live View shooting