Answer
When the pupils are very small, light only passes through the central part of the lens.
Work Step by Step
The chromatic and spherical aberrations are minimal at the center of the lens, so only using the center means that vision will be the sharpest.
Also, the eye acts more like a pinhole camera, and less focusing is required.
This is discussed on page 537, and shown in Figures 28.52 and 28.54.