Canon EOS 30D Instruction Manual page 224

8. Infinity Distance Mark
To compensate for shifting of the infinity focus point that results from changes in
temperature.
To ensure accurate focus, look through the viewfinder and check the focus before
shooting.
9. Infrared Index
(Not available for all cameras)
The figures in red above the distance scale are the infrared index. 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.
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