Canon EF 135mm f2L USM - Manual (Page 7)

ENG-6
6. Depth-of-Field 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.
The depth-of-field scale is an approximate
indicator.
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.
5. Infrared Index
(EF200mm f/2.8L II USM • EF300mm f/4L
USM • EF400mm f/5.6L USM)
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.