Suunto Tandem User Manual page 10

Because of an eye condition called heterophoria,
the reading accuracy of some users may be
impaired. Check for this as follows:
Take a reading with both eyes open and then close
the free eye. If the reading does not change
appreciably there is no disalignment of the eye
axes, and both eyes can be kept open. Should there
be a difference in the readings, keep the other eye
closed and sight halfway above the instrument
body. The hairline now rises above the instrument
body and is seen against the target (Fig. 5).
N
S
View
Fig. 5. The hairline is seen against the target
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