Biomedical Engineering Reference
In-Depth Information
as the distance changes, the linear depth of field depends on the distance: at
40.00 cm it is 8.08 cm, and at 100 cm it is 53.33 cm.
Let's do another problem.
When looking through her bifocal add, a presbyopic
patient is able to see clearly from 25.00 to 100.00 cm. Her depth of field is
±
0.50 D (i.e., the total depth of field is 1.00 D). What is her true amplitude of
accommodation? What is the power of the add she is wearing?
By
true (or actual) amplitude of accommodation
, we are referring to the
change in the eye's dioptric power that is solely due to accommodation. In the cur-
rent example, the patient's range of clear vision, expressed in diopters, is from 4.00
to 1.00 D. The
apparent amplitude of accommodation
, which takes into account
the depth of field, is 3.00 D (i.e., 4.00 - 1.00 D
3.00 D). Since 1.00 D of this
apparent amplitude of accommodation is due to depth of field, the
true
amplitude
of accommodation is 2.00 D.
For a distance-corrected presbyopic patient to resolve near objects, plus power
must be added to the distance prescription. As we learned in Chapter 8, the plus
power that is added to the distance correction is called an
add
(Fig. 11-9). When
=
A
B
Figure 11-9.
A.
Flat-top and round bifocal adds. Not shown is a variable-focus lens,
often called a progressive lens. In such a lens, the plus power gradually increases
going from the center of the lens toward its bottom.
B.
Cross sections of flat-top and
round bifocal lenses. The adds in these glass bifocals have a higher index of refraction
than the distance carrier lenses.
