Fluid Lenses - Geometrical and Visual Optics - page 331

Figure C-1B shows that the fluid lens has a power of + 1.00 DS. Therefore, to

correct the myopia, the contact lens must have a power of − 4.00 DS. 2

What power would be required to correct the myopia if the contact lens has a
base curve of 43.00 D?

Since the front and back surfaces of the fluid lens have the same curvature, the
fluid lens has no power (Fig. C-1C). Consequently, a -3.00 DS contact lens is
required.

Let's consider another example. A patient's eye has the following K readings and
refraction as determined in the corneal plane:

44.00/090
43.00/180
− 5.00 − 1.00 × 180

Using the terminology of Chapter 9, diagnose the patient's refractive error. If
a rigid contact lens with a base curve of 43.00 D is fitted to this cornea, what
power is required to correct the patient's ametropia?

The first step, as illustrated in Figure C-2A, is to draw crosses that show the
prescription (i.e., the required correction) and patient's corneal powers (i.e.,
K readings). The former illustrates that both meridians of the eye are myopic and
that the vertical meridian is more powerful than the horizontal meridian. The
patient has compound myopic with-the-rule astigmatism. The K readings tell us
that the cornea is the source of the astigmatism (rather than the crystalline lens).
A cornea that has cylindrical power is said to be toric .
For a base curve of 43.00 D, what is the power of the fluid lens? From Figure
C-2B, we see that fluid lens has a power of -1.00 D in its vertical meridian and no
power in its horizontal meridian. Therefore, a -5.00 DS contact lens in combina-
tion with the fluid lens will fully correct the refractive error.

If the contact lens has a base curve of 44.00 D, what power is required to correct
the ametropia?

2. The mnemonic SAM FAP , meaning if the lens is s teep to a dd m inus to its power and if f lat to a dd
p lus to its power, is handy to memorize.