If two lenses are ground with 2D of base down prism in each lens, how will the patient perceive this?

When two lenses are ground with 2 diopters (D) of base down prism in each lens, the total effect of the prisms on the patient's perception is important to consider. In this case, each lens has a base down prism that, when positioned in front of the eyes, would individually direct light downward. However, because both lenses are producing the same base down effect, the patient’s brain processes these two opposing effects.

When the prisms in both lenses are aligned and both have the same base down direction, the effects actually counteract each other to some degree when viewed as a combined system. As a result, the patient will perceive minimal to no overall prism effect because the downwards deviation is neutralized by the equal and opposite action of each lens.

Understanding that the prisms are additive or subtractive based on their orientation with respect to one another is key here. In this scenario, instead of a strong perceptible prism effect or any unusual convergence effect, the patient perceives a state where the light is not deviated significantly, leading to an overall perception of no prism.