Biomedical Engineering Reference
In-Depth Information
Positive lens
Object
(printed material)
Figure 12-4.
Stand magnifiers are designed so that the reading material is at a fixed
distance from a plus lens. This distance is often less than the focal length of the lens.
or accommodate to see it clearly. Locating this virtual image allows us to determine
where the patient should position her eye and what the accommodative demand
will be.
Let's consider an example.
A patient can read 4 M print when it is held 40.00 cm
from his spectacles. You prescribe a stand magnifier that contains a
20.00 D
lens. The lens is at a fixed distance of 4.00 cm from the reading material. How
close must the patient's eye be to the plus lens for him to read 1 M print?
+
The first step is to determine the equivalent viewing distance. Since the patient
can currently read 4 M print (at the reference distance of 40.00 cm) and wishes to
read 1 M print, the required angular magnification is 4
. The equivalent viewing
distance, which is four times nearer than the reference distance, is 10.00 cm.
Next, we need to locate the virtual image produced by the stand magnifier and
determine its lateral magnification. In Figure 12-5, we have redrawn the stand mag-
nifier so that it is sitting on its side. A stand magnifier is, of course, not typically
so positioned, but drawing it this way allows us to use our linear sign conventions.
Since the object is located within the focal length of the lens, a virtual image is
formed. The image vergence is
×
′
=
L
L
+
F
(100)(1.00)
⎥
⎦
⎥
⎣
′
=
L
+
20.00 D
−
4.00 cm
′
= −
L
25.00 D
+
20.00 D
′
= −
L
5.00 D
