What is the minimal pupil size required for a quality OCT scan?

• A. 2 mm
• B. 3 mm
• C. 4 mm
• D. 5 mm

Answer: (B)

The optic signal in OCT relies on how much light you can couple into and out of the eye. The pupil acts as the entrance aperture; its size determines how much light reaches the retina and how much backscattered light the system can collect. A pupil large enough provides a higher numerical aperture, which improves both the signal strength and the lateral resolution. If the pupil is too small, the OCT signal weakens, noise rises, and fine retinal details can be blurred or lost, especially in regions farther from the fovea or under suboptimal fixation. Therefore, about 3 mm is the minimum pupil diameter that typically yields a quality, reliable OCT scan with standard equipment. Smaller pupils tend to degrade image quality, while larger pupils can further enhance performance, but 3 mm is the practical lower limit for obtaining good scans.