Canon EF-S 17-55 f/2.8 IS USM EF-S17-55mm F2.8 IS USM Instruction Manual - Page 6

3 Zooming 4 Infinity Compensation Mark Infinity Compensation Mark To zoom, turn the lens' zoom ring. Be sure to finish zooming before focusing. Changing the zoom ring after focusing can affect the focus. 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. ENG-5