Biomedical Engineering Reference
In-Depth Information
A
F
= -6.68 D
n
= 1.000
n
′
= 1.333
FP
-14.97 cm
B
F
= -7.42 D
n
= 1.000
n
′
= 1.333
FP
-1.50 cm
-13.47 cm
Figure 7-8.
This refractive error can be corrected with (
A
) a -6.68 D contact lens or
(
B
) a -7.42 D spectacle lens. In both cases, an infinitely distant object is focused at
the eye's far point.
This myopic eye can be corrected with either a -6.68 D contact lens or a -7.42 D
spectacle lens at a vertex distance of 15.00 mm. Both have the same
effective
power.
Note that when we replace a myopic patient's contact lens with a spectacle lens, the
power must be increased. Think of it this way: since we are moving the corrective
lens closer to the far point, it must now have a shorter focal length.
Understanding lens effectivity is critical to understanding and solving com-
mon clinical symptoms. For instance, patients with myopia sometimes report that
blurred distant objects become clearer when they push their spectacles very close
to their eyes. How can we explain this? Most likely, the corrective lenses are not
sufficiently strong. As illustrated in Figure 7-9, by pushing the lenses closer to the
eyes, the secondary focal points of the corrective lenses are made coincident with
the far points, thereby increasing the lens effectivity.
