Canon EF 180mm f/3.5L Macro USM EF180mm F3.5L MACRO USM Instruction Manual - Page 7

5. Infinity Compensation Mark Infinity compensation mark 6. Depth-of-Field Scale 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. The depth of field is the distance in front of and behind the plane of focus on the subject that appears sharp. The depth of field is indicated by the area between the depth-of-field scale lines below the distance scale. The number on the scale is the F value. For accurate manual focusing on subjects at infinity distance, look through the viewfinder while rotating the focusing ring. The depth-of-field scale is an approximate indicator. ENG-6