Biomedical Engineering Reference
In-Depth Information
looking through the add in the current example, the nearest clear distance (i.e., the
near point of accommodation) is 25.00 cm in linear units or 4.00 D in dioptric units.
Of this 4.00 D, 2.00 D is due to accommodation and 0.50 D to the depth of field.
The add provides the remaining power. Hence, the add has a power of 4.00 - 2.00 -
0.50 D
=
1.50 D. You'll need to study Figure 11-10A to put this all together.
A
+1.50 DS
-100.00 cm
or
-1.00 D
-66.67 cm
or
-1.50 D
-28.57 cm
or
-3.50 D
-25.00 cm
or
-4.00 D
0.50 D
0.50 D
A
B
2.00 D
Accommodation
3.00 D
Range of clear vision
B
+1.50 DS
-100.00 cm
or
-1.00 D
-66.67 cm
or
-1.50 D
0.50 D
Figure 11-10.
A.
A presbyopic patient, fully corrected for distance and viewing
through her add, has a range of clear vision that extends from
−
25.00 (or
−
4.00 D) to
−
1.00 D). If the patient did not have a 1.00 D depth of field, her range
of clear vision would be from
A
(-1.50 D or − 66.67 cm) to
B
(-3.50 D or −28.57 cm);
therefore, the true amplitude of accommodation (as measured at the spectacle plane)
is 2.00 D. Because the patient's NPA is 25.00 cm, she must be wearing a 1.50 add (i.e.,
0.50
100.00 cm (
−
+
2.00
+
1.50 D
=
4.00 D).
B.
Since the farthest distance the patient can see is
−
100.00 cm and the depth of field is 1.00 D (i.e.,
±
0.50 D), the patient must be wearing
a
+
1.50 DS add. See the text for details.
