Originally Posted by
Robert Martellaro
Oscar,
The in and out induced prism is not so much of a concern compared to up and down vertical imbalance (VI).
Most astigmatism, excluding corneal disorders and surgery, have mirror symmetries in their cylinder axes, where the axes sum to 180 or 90 degrees, minimizing VI. For example,
OD pl -2.00 x 60
OS pl -2.00 x 120
induces 0.00 VI at the PRP or reading depth.
When the axes do not mirror each other,
OD pl -2.00 x 60
OS pl -2.00 x 60
more than 1∆ VI is induced at a 12mm reading level and less than half that at a PRP with a 4mm drop. Likely tolerated. Separates might be best for higher degrees of VI.
Hope this helps,
Robert Martellaro
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