Why should the optic zone diameter be larger than the pupil by 1 mm?

Why should the optic zone diameter be larger than the pupil by 1 mm?

• A. Can cause glare/haloes
• B. Can improve comfort
• C. Reduces movement
• D. Increases tear exchange

Answer: (C)

The main idea is to keep the pupil well inside the central corrected area of a gas-permeable lens so vision stays stable as the eye moves or the pupil changes size. When the optic zone is larger than the pupil by about 1 mm, even with natural blinking and occasional pupil dilation, the pupil remains largely within the central, fully corrected zone. This reduces the influence of edge effects and small shifts of the lens on vision, so the image on the retina stays steadier and less sensitive to movement. Other options relate to outcomes that are less directly dependent on optic zone size. Glare and halos can occur if the pupil overlaps the edge, but the primary aim of making the optic zone larger than the pupil is to maintain stable optical alignment and reduce the impact of movement. Comfort is more about fit and surface properties, while tear exchange is governed by tear film dynamics and lens fit, not the optic zone diameter itself.

The main idea is to keep the pupil well inside the central corrected area of a gas-permeable lens so vision stays stable as the eye moves or the pupil changes size. When the optic zone is larger than the pupil by about 1 mm, even with natural blinking and occasional pupil dilation, the pupil remains largely within the central, fully corrected zone. This reduces the influence of edge effects and small shifts of the lens on vision, so the image on the retina stays steadier and less sensitive to movement.

Other options relate to outcomes that are less directly dependent on optic zone size. Glare and halos can occur if the pupil overlaps the edge, but the primary aim of making the optic zone larger than the pupil is to maintain stable optical alignment and reduce the impact of movement. Comfort is more about fit and surface properties, while tear exchange is governed by tear film dynamics and lens fit, not the optic zone diameter itself.