ACCOMMODATION

Definition. As we know that in an emmetropic eye,
parallel rays of light coming from infinity are brought
to focus on the retina, with accommodation being at
rest. However, our eyes have been provided with a
unique mechanism by which we can even focus the
diverging rays coming from a near object on the retina
in a bid to see clearly (Fig. 3.32). This mechanism is
called accommodation. In it there occurs increase in
the power of crystalline lens due to increase in the
curvature of its surfaces (Fig. 3.33).
At rest the radius of curvature of the anterior
surface of the lens is 10 mm and that of posterior
surface is 6 mm (Fig. 3.33A). In accommodation, the
curvature of the posterior surface remains almost the
same, but the anterior surface changes, so that in
strong accommodation its radius of curvature
becomes 6 mm (Fig. 3.33B).
Mechanism of accommodation
According to von Helmholtz capsular theory in
humans the process of accommodation is achieved
by a change in the shape of lens as below:
When the eye is at rest (unaccomodated), the
ciliary ring is large and keeps the zonules tense.
Because of zonular tension the lens is kept
compressed (flat) by the capsule (Fig. 3.33A).
Contraction of the ciliary muscle causes the
ciliary ring to shorten and thus releases zonular
tension on the lens capsule. This allows the
elastic capsule to act unrestrained to deform the
lens substance. The lens then alters its shape to
become more convex or conoidal (to be more
precise) (Fig. 3.33B). The lens assumes conoidal
shape due to configuration of the anterior lens
capsule which is thinner at the center and thicker
at the periphery (Fig. 3.33).
Far point and near point
The nearest point at which small objects can be seen
clearly is called near point or punctum proximum
and the distant (farthest) point is called far point
or punctum remotum.
Far point and near point of the eye vary with the
static refraction of the eye (Fig. 3.34).
In an emmetropic eye far point is infinity (Fig.
3.34A) and near point varies with age.
In hypermetropic eye far point is virtual and lies
behind the eye (Fig. 3.34B).
In myopic eye, it is real and lies in front of the
eye (Fig. 3.34C).
Range and amplitude of accommodation
Range of accommodation. The distance between the
near point and the far point is called the range of
accommodation.
Amplitude of accommodation. The difference
between the dioptric power needed to focus at near
point (P) and far point (R) is called amplitude of
accommodation (A). Thus A = P – R.
ANOMALIES OF ACCOMMODATION
Anomalies of accommodation are not uncommon.
These include: (1) Presbyopia, (2) Insufficiency of
accommodation, (3) Paralysis of accommodation, and
(4) Spasm of accommodation.