Biomedical Engineering Reference
In-Depth Information
and bifocal add is less than the focal length of the magnifying lens, the patient
experiences greater angular magnification (than when the material is held at the
focal point of the lens while looking through the distance correction). In the next
section, we take a slightly different approach to this issue.
Magnifying Lens and Bifocal Add in Combination
Will a patient obtain more magnification when looking through a handheld mag-
nifying lens with her distance prescription (keeping the object at the primary
focal point of the magnifying lens) or with her bifocal add (keeping the object at
the primary focal point of the add-magnifier combination)? As we will see from
the following examples, it depends on how far the magnifier is held from the
add. Consider a patient who wears bifocals that have a 2.50 D add. A
+
10.00 D
handheld magnifying lens (i.e.,
F
e
=
+
10.00 D) is prescribed to assist the patient
with reading.
Suppose the patient holds the magnifying lens 10.0 cm from her add (i.e., the
separation between the magnifier and add is equal to the focal length of the magni-
fier). What is the equivalent power of this magnifier-add combination? To answer
this question, we treat the magnifier-add combination as a thick lens. Using the
thick lens formula from Chapter 6, we have
t
⎛
⎞
⎠
F
e
=
F
1
+
F
2
−
F
1
F
2
⎝
⎛
⎝
0.1 m
⎞
⎠
F
e
= +
2.50 D
+
10.00 D
−
(
+
2.50 D)(
+
10.00 D)
1.00
F
e
= +
10.00 D
When the magnifier is held one focal length from the add, the magnifier-add
combination has the same equivalent power as the magnifier itself. Consequently,
the same angular magnification is obtained whether the patient looks through the
magnifier with her add or distance prescription.
Now suppose the patient holds a magnifying lens less than 10.0 cm from her add,
say at 5.0 cm from her add. What is the power of the system?
⎛
t
⎞
⎠
F
e
=
F
1
+
F
2
−
F
1
F
2
⎝
0.05 m
⎛
⎝
⎞
⎠
F
e
= +
2.50 D
+
10.00 D
−
(
+
2.50 D)(
+
10.00 D)
1.00
F
e
= +
11.25 D
When the distance between the magnifier and add is less than the focal length
of the magnifier, the equivalent power of the magnifier-add combination is greater
than that of the magnifier. Consequently, the angular magnification is greater when
the patient looks through the magnifying lens with her add than when she looks
