Human Physiology, 14th edition (2016)

296 Chapter 10

influencing acuity), the line of letters marked “20/20” can be

read ( fig.  10.35 a ). If a person has myopia (nearsightedness),

this line will appear blurred because the image will be brought

to a focus in front of the retina. This is usually due to an eye-

ball that is too long. Myopia is corrected by glasses with con-

cave lenses that cause the light rays to diverge, so that the point

of focus is farther from the lens and is thus pushed back to the

retina ( fig. 10.35 b ).

If the eyeball is too short, the line marked “20/20” will

appear blurred because the focal length of the lens is longer

than the distance to the retina. Thus, the focus of the image

would have been behind the retina, and the object will have to

be placed farther from the eyes to be seen clearly ( fig. 10.35 c ).

This condition is called hyperopia (farsightedness). Hypero-

pia is corrected by glasses with convex lenses that increase

the convergence of light, so that the point of focus is brought

closer to the lens and falls on the retina.

reading (magnifying) lenses to clearly see small objects at
close distances.

Visual Acuity

Visual acuity refers to the sharpness of vision. The sharpness of
an image depends on the resolving power of the visual system—
that is, on the ability of the visual system to distinguish (resolve)
two closely spaced dots. The better the resolving power of the
system, the closer together these dots can be and still be seen as
separate. When the resolving power of the system is exceeded,
the dots blur and are perceived as a single image.

Myopia and Hyperopia

When a person with normal visual acuity stands 20 feet from
a Snellen eye chart (so that accommodation is not a factor

Figure 10.34 Changes in the shape of the lens permit accommodation. ( a ) The lens is flattened for distant vision when
the ciliary muscle fibers are relaxed and the suspensory ligament is taut. ( b ) The lens is more spherical for close-up vision when the
ciliary muscle fibers are contracted and the suspensory ligament is relaxed.

Ciliary muscle

fibers relaxed

Suspensory

ligament taut

Lens thin

and focused

for distant

vision

(a)

(b)

Ciliary muscle

fibers contracted

Suspensory

ligament

relaxed

Lens thick

and focused

for close

vision