Biomedical Engineering Reference
In-Depth Information
Because the eye is looking through the lower portion of a meridian that has
plus power, the patient experiences 0.60
Δ
base up.
8. (100)(1.00)/
7.69 D. The prism does not change the vergence of
the light rays that are incident upon it.
−
13.00 cm
= −
CHAPTER 11
Depth of Field
1. Since the hyperfocal distance is 200.00 cm, the total depth of field is 1.00 D.
If the near point of accommodation is 10.00 cm, the true amplitude of accom-
modation is 9.50 D (i.e., 10.00
−
0.50 D
=
9.50 D).
2. First, we calculate the true amplitude of accommodation. The patient's range
of clear vision (at near) corresponds to a dioptric range of 1.00 to 4.00 D. Since
the depth of field is 1.00 D, the true amplitude of accommodation is 2.00 D
(i.e., 3.00
2.00 D). When looking through the distance prescrip-
tion, the patient can accommodate 2.00 D. If the depth of field was zero, the
patient could not resolve objects nearer than 50.00 cm. The near boundary
of the patient's range of clear vision, however, is extended by one-half of the
depth of field (i.e., 0.50 D). Therefore, the range of clear vision through the
distance portion of the spectacles is from infinity to 40.00 cm (i.e., 2.50 D).
[Due to depth of field, when the patient is focused at 50.00 cm (2.00 D) she can
see clearly at 40.00 cm (2.50 D).]
−
1.00 D
=
3. The far point for an uncorrected 1.00 D myope is at
1.00 D. Due to depth of
field, which is centered at a distance of 100.00 cm, the patient can see clearly
out to a distance of 0.25 D or 400.00 cm (i.e., 1.00
−
−
0.75 D
=
0.25 D).
4. Through the add, the patient can see clearly out to 50.00 cm. Since the
depth of field is 0.50 D, the patient's add has a power of
+
2.25 D (i.e., 2.00
+
0.25 D
2.25 D lens focuses the eye at
44.44 cm, the patient's depth of field allows her to see out to 50.00 cm.
=
2.25 D). In other words, when a
+
Another approach to solving this problem is to first determine the true
amplitude of accommodation. The dioptric range of clear vision is from 2.00
to 5.00 D. Since 0.50 D of this range is due to depth of field, the patient's
amplitude of accommodation is 2.50 D. If there were no depth of field, the
patient would need an add of 2.50 D to see at 20.00 cm (5.00
−
2.50 D
=
2.50 D). Taking into account depth of field, however, we know the power of
the patient's add is only 2.25.
5. (a) The true amplitude of accommodation is
(5.00 D
−
2.50 D)
−
0.75 D
=
1.75 D
