Pupillary Diameter The major function of the iris is to increase the amount of light that enters
the eye during darkness and to decrease the amount of light that enters the eye in daylight.
Figure 49-10 Mechanism of accommodation (focusing).Unit X The Nervous System: B. The Special Senses 602
The amount of light that enters the eye through the pupil is proportional to the area of
the pupil or to the square of the diameter of the pupil.
Figure 49-11 Effect of small (top) and
large (bottom ) papillary apertures on “depth of focus.”
Emmetropia Hyperopia Myopia Figure 49-12 Parallel light rays focus on the retina in emmetropia, behind the retina in hyperopia,
and in front of the retina in myopia. Chapter 49 The Eye: I. Optics of Vision 603
the person can
use the mechanism of accommodation to keep the image focused clearly. A myopic person has a
definite limiting ―far point‖ for clear vision.
