What is the actual ortho separation in Brewster distance?

• A. 87 mm
• B. 95 mm
• C. 68 mm
• D. 63 mm

Answer: (A)

At Brewster distance, the way the two eyes point to the same fixation target creates a specific spatial separation between the two visual axes on that fixation plane—the ortho separation. This separation isn’t just the raw interpupillary distance; it depends on how the eye centers project through the geometry of the eyes to the plane at that depth. In practice, for a typical adult with a normal IPD and head geometry, this Brewster-stage geometry yields an ortho separation around 87 mm, which is why that option is the best choice. The exact value can vary a bit with individual IPD and head shape, but the Brewster-distance setup tends to produce a larger ortho separation than the simple IPD alone.