Canon Model 7S - Manual (Page 63)

Infrared Photography
The letter "R” is on the depth of-field scale of
all Canon lenses. This is for infrared film. When
using this film, focus in the normal way. Read
off the distance of the subject you are focusing
on as shown opposite the distance indicator mark
on the lens distance scale. Turn the lens barrel
until the distance reading is opposite the "R”
mark. For example, when the subject you have
focused on is 25-feet away, turn the lens barrel
until the figure 25 is opposite the “R” mark.
Your lens is now focused for infrared photography.
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