American Board of Opticianry (ABO) Practice Test

Which combination of prism is not a compounding situation?

1. Base In and Base In

2. Base Out and Base Out

3. Base Up and Base Up

4. Base Down and Base Down

The correct answer is: Base Down and Base Down

In the context of prism combinations, compounding refers to the process where the effects of multiple prisms in the same direction add together, effectively increasing the total prismatic effect. When prisms are aligned with the same base direction, they work together to create a larger deflection of light. The combination of prisms with bases in the same direction, such as Base In with Base In, Base Out with Base Out, or Base Up with Base Up, results in a compounded effect because they amplify the original prismatic strength. However, the combination of Base Down and Base Down does not create a compounding situation in the same way. When two prisms are oriented with their bases down, they do not effectively increase the overall prismatic effect in a useful way for optical correction or alignment because they simply reinforce a downward deviation rather than creating a balanced visual result. This can lead to issues such as overcorrection or insufficient alignment rather than a meaningful compounding of the prism's benefits. Understanding these principles helps opticians and eye care professionals utilize prisms effectively and select the appropriate combinations for their patients' needs.