Which statement best describes the role of the VA ratio in the Lighthouse ADD calculation?

Which statement best describes the role of the VA ratio in the Lighthouse ADD calculation?

• A. It is added to the working distance.
• B. It is multiplied by the working distance.
• C. It is divided by the working distance.
• D. It is ignored in the calculation.

Answer: (B)

The key idea is that the VA ratio acts as a scale factor for turning how far the target is held into the additional near power the patient needs. In the Lighthouse ADD method, the VA ratio (which reflects how much acuity changes from distance to near) is multiplied by the working distance to yield the near add in diopters. This makes sense because both a longer working distance and a larger drop in acuity toward near increase the needed add, and they combine proportionally through multiplication. If you used addition, division, or ignored this ratio, you wouldn’t capture how the distance and acuity change together to determine the appropriate near correction.

The key idea is that the VA ratio acts as a scale factor for turning how far the target is held into the additional near power the patient needs. In the Lighthouse ADD method, the VA ratio (which reflects how much acuity changes from distance to near) is multiplied by the working distance to yield the near add in diopters. This makes sense because both a longer working distance and a larger drop in acuity toward near increase the needed add, and they combine proportionally through multiplication. If you used addition, division, or ignored this ratio, you wouldn’t capture how the distance and acuity change together to determine the appropriate near correction.