Surgery and Healing in the Developing World - Dartmouth-Hitchcock

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342 Surgery and Healing in the Developing World
Figure 5. Equipment.
Axis Measurement
If required the axis of the lens can be determined. This can be achieved by using
the exact opposite lens from your optometrist’s set (i.e., if the patient’s lens is +2.5,
select a -2.5 lens from your set with the axis premarked). The two lenses should then
be positioned and rotated until the axes align (this is when the overall effect of the
two lenses is zero. A simple check is to focus on an object through both lenses then
move them back and forth by approximately 5 cm. This should not distort your
focus if they are correctly aligned). Once the axis is determined it should be marked
as two points on either side of the lens.
Axis Examination
Position the lens on a standard axis chart. Any deviation from the standard axis
(due to astigmatism) should be marked as a line across the lens (Fig. 6).