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 manual focusing of subjects at infinity, look through the viewfinder or look at the magnified image* on the LCD screen while rotating the focusing ring.   * For cameras with Live View shooting capability.