Suunto TANDEM Guide De L'utilisateur 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
Code:
PM-5/360PC
closed and sight halfway above the instrument
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body. The hairline now rises above the instrument
body and is seen against the target (Fig. 5).
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TANDEM
View
190 180 180
10 0 350
Fig. 5. The hairline is seen against the target
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