When analyzing prism in a PAL, which approach should be used to determine the total prism for both eyes?

• A. Sum of left and right individually
• B. Paired prism rules
• C. Do not consider prism
• D. Only vertical prism + horizontal prism

Answer: (B)

When analyzing prism in a PAL, it’s essential to treat the prisms as a binocular effect rather than as separate prisms for each eye. The paired prism rules let you combine the horizontal and vertical components from both eyes to find the overall prism the wearer experiences.

Think of prism as a vector: it has direction (base direction) and magnitude. If both eyes have prisms with bases in the same direction, those effects add to give a larger net prism. If the bases point in opposite directions, they cancel to the extent of their magnitudes. Vertical components behave similarly, adding when the directions match and subtracting when opposite. This approach reflects how the two eyes work together to fuse images.

Using these rules ensures you account for how prisms interact binocularly, which is why it’s the best method. Simply summing prisms eye by eye ignores the fusion and the potential cancellation or enhancement that occurs between the eyes. Not considering prism altogether or treating horizontal and vertical parts separately without a paired framework would miss how the two eyes influence each other in a PAL.