Nikon FG Instruction Manual page 41

The scale on the ASA / ISO dial has numbered settings
for speeds from ASA/ISO 12 to 3200 . Two dots be-
tween each number stand for intermediate settings ,
such as 64 , 80 , etc _ The 64 , 100 and 400 settings are
indicated in red _ The table gives the speeds for all
intermediate settings, To set the film speed in use, lift
up the dial and rotate it until the desired number (or
dot representing the film speed) click-stops opposite
the inde x dot 5.1; _
ASAI ISO is a numerical rating of the film's sensitivity
to a given amount of light. The higher the number, the
greater the sensitivity, and vice versa _ The film 's ASAI
ISO is indicated on the cartridge itself, on the film
carton and on the data sheet packed inside .
This dial can also be used to compensate exposure,
as in the following example : with ASA/ISO 100 film,
set the dial to 50 to overexpose
+
1 step ; to 25 for
+
2 steps ; to 200 for -1 step; to 400 for -2 steps .
After compensated shootings, be sure to return the
dial to the film 's original speed
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