Biomedical Engineering Reference
In-Depth Information
or
0.00 D
-100.00 cm
or
-1.00 D
-50.00 cm
or
-2.0
0
D
1.00 D
1.00 D
Figure 11-12.
When taking pilocarpine, which gives the patient a 2.00 D depth of field,
the patient can have up to 1.00 D of uncorrected myopia and still have 20/20 acuity.
See the text for details.
(1.00 D), the depth of field extends to infinity, making 20/20 vision possible. Hence,
the patient can have up to 1.00 D of myopia and still have an uncorrected distance
visual acuity of 20/20. (If the patient discontinues pilocarpine and the pupil returns
to its normal size, the depth of focus will be less than
±
1.00 D and the distance
uncorrected acuity will, therefore, be less than 20/20.)
Let's try one more problem.
An emmetropic patient with absolute presbyopia has
a hyperfocal distance of 200.00 cm. What is the range of clear vision in linear
units when the patient wears
+
2.50 DS reading glasses?
The patient's hyperfocal distance is 200.00 cm in linear units or 0.50 D in dioptric
units, making the depth of field 1.00 D
.
When wearing the
2.50 DS reading glasses,
the depth of field is centered at 2.50 D (40.00 cm). As can be seen in Figure 11-13, the
depth of field extends from 2.00 to 3.00 D, or from 50.00 to 33.33 cm.
+
+2.50 DS
-50.00 cm
or
-2.00 D
-40.00 cm
or
-2.50 D
-33.33 cm
or
-3.00 D
0.50 D
0.50 D
Figure 11-13.
Since the hyperfocal distance is
200.00 cm, the depth of field is 1.00 D.
When this emmetropic patient with absolute presbyopia wears
−
+
2.50 DS reading
glasses, the range of clear vision is from
−
50.00 cm (
−
2.00 D) to
−
33.33 cm (
−
3.00 D).
