3. Infinity Compensation Mark 4. Infrared Index
Infinity compensation mark
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.
The infrared index corrects the focus setting when using monochrome infrared film. Focus on the subject manually, then adjust the distance setting by moving the focusing ring to the corresponding infrared index mark.
Some EOS cameras cannot use infrared film. See the instructions for your EOS camera.
¡The infrared index position is based on a wavelength of 800 nm.
¡Be sure to observe the manufacturer’s instructions when using infrared film.
¡Use a red filter also when you take the picture.