Biomedical Engineering Reference
In-Depth Information
Consider three 50-year-old patients. Each has an amplitude of accommodation
of 2.00 D. One patient has emmetropia, another has 2.00 D of uncorrected
myopia, and the third has 2.00 D of uncorrected hyperopia (all measured in the
corneal plane). Determine the NPA for each patient.
First consider the emmetropic eye (Fig. 8-8A). The amplitude of accommodation
is 2.00 D and the far-point vergence is zero (i.e., the far point is at infinity). From
the vergence relationship for accommodation, we have
F
FP
=
L
+
F
A
0.00
=
L
+
2.00
L
= −
2.00 D
When the eye is maximally accommodated, objects located as close as 50.00 cm
(100/
50.00 cm) in front of the cornea can be imaged on the retina
(i.e., the NPA is 50.00 cm). An object that is closer than 50.00 cm, however, cannot
be imaged on the retina because the eye does not have sufficient accommodative
power. For instance, for an object located 40.00 cm from this eye to be imaged
on the retina, the eye would need to accommodate 2.50 D. Since the eye has only
2.00 D of accommodation, the image is blurred.
Now, let's determine the NPA for the uncorrected 2.00 D myopic eye (Fig. 8-8B).
The maximum amount of accommodation is the same as it is for the emmetropic
eye (2.00 D), but the far-point vergence is -2.00 D. Therefore,
−
2.00 D
=
−
F
FP
=
L
+
F
A
−
2.00
=
L
+
2.00
L
= −
4.00 D
For this uncorrected myopic eye, the NPA is 25.00 cm: objects located as close as
25.00 cm anterior to the eye can be imaged on the retina. For the emmetropic eye,
an object at 25.00 cm will be out of focus. The uncorrected myopic eye is better
able to focus near objects than the emmetropic eye.
Now it's time to look at a 2.00 D uncorrected hyperopic eye that also has an
amplitude of accommodation of 2.00 D. Substituting in the vergence relationship
for accommodation, we have
F
FP
=
L
+
F
A
+
2.00
=
L
+
2.00
L
=
0.00 D
); consequently, when this hyper-
opic patient is uncorrected, any object closer than optical infinity will be out of
focus (Fig. 8-8C). With the same amplitude of accommodation, an emmetrope has
an NPA of 50.00 cm and an uncorrected 2.00 D myope has an NPA of 25.00 cm.
The uncorrected myope has an advantage when viewing near objects, whereas the
The NPA is at infinity (100/0.00 D
=
∞
