Biomedical Engineering Reference
In-Depth Information
SUMMARY
For near objects to be clearly imaged on the retina, the eye's crystalline lens must
increase in power, a process referred to as accommodation. Through accommoda-
tion, near objects are imaged at the far point of the eye, which is conjugate with the
retina.
Since the uncorrected myopic eye is too strong for its length, it requires less
accommodation to image near objects on the retina than the uncorrected hyperopic
eye, which is too weak for its length. The manner in which ametropia is corrected
determines the amount of accommodation that is required. A myopic eye corrected
with a spectacle lens accommodates less than when corrected with a contact lens.
In comparison, the hyperopic eye must accommodate more when corrected with a
spectacle lens compared to a contact lens.
As we age, there is a decrease in the accommodative amplitude. When this pro-
duces symptoms such as near blur and asthenopia, the condition is referred to as
presbyopia. Presbyopia may be corrected by adding plus power to the distance
prescription.
KEY FORMULA
Vergence relationship for accommodation:
F
FP
=
L
+
F
A
SELF-ASSESSMENT PROBLEMS
6
1. An object is located 10.00 cm anterior to the cornea of a 3.50 D myopic eye
(as measured at the corneal plane). How much accommodation is required to
image the object on the retina if (a) the eye is uncorrected? (b) corrected with a
contact lens? and (c) corrected with a spectacle lens? Assume a vertex distance of
15.00 mm.
2. A 5.00 D hyperopic eye (as measured in the corneal plane) views an object that is
located 10.00 cm anterior to his cornea. If the eye is wearing a
4.00 DS contact
lens, how much accommodation is required to image the object on the retina?
3. An emmetropic presbyopic patient has a range of clear vision from infinity to
20.00 cm from the cornea. What power contact lens would allow the patient
to see clearly at 15.00 cm when using only one-half of her total amplitude of
accommodation? (Ignore depth of field.)
4. To image an object located 33.00 cm anterior to its cornea onto its retina, an
uncorrected 2.00 D hyperopic eye (as measured in the corneal plane) exerts one-
half of its accommodative amplitude. What is the eye's amplitude of accommo-
dation? (Ignore depth of field.)
+
6. In solving these problems, assume that accommodation is measured in the corneal plane.
