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How to stay afloat in the lacrimal pool

1 May 1995


Compensating for a diagnostic RGP contact lens fit flatter or more steeply than the cornea when ordering a prescription has been a concern of optometrists since the invention of rigid contact lenses. Traditionally, the convention of considering the lacrimal lens induced power has been a means of attaining the compensating power when a lens is ordered. The convention is SAMFAP, steep add minus, flat add plus. When a lens is fitted steeply, minus must be added to the ordered lens, and the converse for a flat lens. While this has usually resulted in a satisfactory lens for the patient, the reason for the convention is incorrect and has created a misunderstanding for those prescribing. The lacrimal lens induced by a steeply fit contact lens is not a plus lens as understood by most students but is really a minus lens. While the lacrimal lens is a useful memory device, it should not be used as an explanation for adding minus power to a steeply fit rigid contact lens. Furthermore, the convention overestimates the amount of minus to be ordered by 0.31 D for every diopter that it is fitted steep.


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