In Brewster stereoscope near setting, which is true?

In Brewster stereoscope near setting, which is true?

• A. Targets 13.3 cm away; 2.00 D accommodation; 58 mm separation.
• B. Targets 13.3 cm away; 0 D accommodation; 87 mm separation.
• C. Targets 20 cm away; 0 D accommodation; 87 mm separation.
• D. Targets 13.3 cm away; 2.50 D accommodation; 63 mm separation; For every 1.33 mm change in target separation there will be a 1 prism diopter change in vergence.

Answer: (D)

In the Brewster stereoscope near setting, the visual system is driven by vergence demand set by the physical separation of the two targets and the observer’s IPD, while accommodation is fixed by the instrument’s optics to a specific virtual distance. The near setting typically places targets at about 13.3 cm, but the optical design creates a virtual image at a distance that requires roughly 2.5 D of accommodation. The target separation is chosen to match the observer’s IPD, about 63 mm, so the eyes converge appropriately for fusion. A key practical relation is that changes in target separation translate into vergence changes in a predictable way: approximately 1 prism diopter per 1.33 mm of target separation change. This combination—13.3 cm target distance, about 2.5 D of accommodation, 63 mm target separation, and the 1.33 mm per diopter linkage—fits the Brewster near-setting behavior and explains why that option is correct.

In the Brewster stereoscope near setting, the visual system is driven by vergence demand set by the physical separation of the two targets and the observer’s IPD, while accommodation is fixed by the instrument’s optics to a specific virtual distance. The near setting typically places targets at about 13.3 cm, but the optical design creates a virtual image at a distance that requires roughly 2.5 D of accommodation. The target separation is chosen to match the observer’s IPD, about 63 mm, so the eyes converge appropriately for fusion. A key practical relation is that changes in target separation translate into vergence changes in a predictable way: approximately 1 prism diopter per 1.33 mm of target separation change. This combination—13.3 cm target distance, about 2.5 D of accommodation, 63 mm target separation, and the 1.33 mm per diopter linkage—fits the Brewster near-setting behavior and explains why that option is correct.