Why the correct answer is right
In ophthalmic optics, horizontal prisms are additive when they point in the same direction relative to the nose. Base In (BI) for the right eye (OD) and Base In (BI) for the left eye (OS) both shift the image toward the temple, meaning the eyes must work together in the same way to compensate. Because both lenses are moving the image in a direction that requires the same type of muscular effort, their effects compound to create a larger total prism power.
Why the other options are incorrect
Horizontal prisms only cancel each other out if they are in opposite directions, such as Base In for one eye and Base Out for the other. Vertical imbalance is caused by a difference in vertical prism (Base Up versus Base Down) or significant power differences in the vertical meridian, not by horizontal prism. Prism does not reduce the effective power of a lens; it only changes the perceived location of the object being viewed.
Memory aid
Think of two rowers in a boat, both rowing toward the center (the nose). Since they are both pulling in the same direction toward the middle, their forces combine to move the boat faster. "Same direction, Sum it up" helps you remember that BI/BI or BO/BO combinations always add together.
Real-world application
When a patient has a convergence insufficiency, a doctor might prescribe Base In prism for both eyes to make it easier for the eyes to align. If the prescription says 3 BI OD and 3 BI OS, the patient is effectively looking through a total of 6 prism diopters of Base In. Understanding this compounding effect ensures the optician verifies that the total prism displacement matches the doctor's intent for binocular comfort.
