Fitting
& Verification
Practice Exercises
For ease of reference it is best to print this page before completing
the following optional practice exercises. But first we should make
the following observation for fitting rigid contact lenses according
to the Dyer nomogram. That is the following: flatter base curves call
for lenses with larger diameters, while steeper base curves call for
lenses with smaller diameters. This would stand to reason since larger
corneas are generally flatter while smaller corneas are steeper in curvature.
Using the data found in the following exercises, indicate the lens parameters
by filling in the table below. Refer to the charts and vertex distance
table found on page 9. The answers may be found on the bottom of this
page.
|
1)
|
Spectacle
Rx: -4.75 - 0.50 x 170
Vertex distance = 12 mm
K: 45.50/46.00 |
|
2)
|
Spectacle
Rx: -2.50 - 0.25 x 045
Vertex distance = 8 mm
K: 43.00/43.25 |
|
3)
|
Spectacle
Rx: +3.50 + 1.50 x 90
Vertex distance = 10 mm
K: 46.75/48.25 |
|
4)
|
Spectacle
Rx: +2.75 - 0.75 x 30
Vertex distance = 12 mm
K: 44.37/45.12 |
|
5)
|
Spectacle
Rx: -8.75 - 2.25 x 65
Vertex distance = 8 mm
K: 41.25/43.50 |
|
6)
|
Spectacle
Rx: -5.50 + 1.75 x 120
Vertex distance = 10 mm
K: 43.00/44.75 |
|
7)
|
Spectacle
Rx: +3.75 + 0.50 x 135
Vertex distance = 12 mm
K: 47.75/48.25 |
| Exercise
# |
1
|
2
|
3
|
4
|
5
|
6
|
7
|
Total
power at
Corneal Plane (D)* |
-4.50 |
. |
.
|
.
|
.
|
.
|
.
|
| Base
Curve (D) |
45.75 |
.
|
.
|
.
|
.
|
.
|
.
|
Power
of
Tear Lens (D) |
+0.25 |
.
|
.
|
.
|
.
|
.
|
.
|
Power
of
Contact Lens (D) |
-4.75 |
.
|
.
|
.
|
.
|
.
|
.
|
Overall
Lens Diameter (mm) |
8.1 |
.
|
.
|
.
|
.
|
.
|
.
|
| Optical
Zone (mm) |
6.7 |
.
|
.
|
.
|
.
|
.
|
.
|
| Peripheral
Curve (D) |
42.00 |
.
|
.
|
.
|
.
|
.
|
.
|
| Center
Thickness (mm)** |
0.10 |
.
|
.
|
.
|
.
|
.
|
.
|
* Round off power
to nearest 0.25 diopter
**Use closest diameter
indicated on thickness chart
Answers
to Practice Exercises
| Exercise
# |
1
|
2
|
3
|
4
|
5
|
6
|
7
|
Total
power at
Corneal Plane (D)* |
-4.50 |
-2.50 |
+5.25
|
+2.75
|
-8.25 |
-3.75 |
.+4.50
|
| Base
Curve (D) |
45.75 |
43.25 |
47.50 |
45.00 |
42.25 |
45.75 |
48.00
|
Power
of
Tear Lens (D) |
+0.25 |
+0.25 |
+0.75 |
+0.63 |
+1.00 |
+0.75 |
+0.25 |
Power
of
Contact Lens (D) |
-4.75 |
-2.75 |
+4.50 |
+2.12 |
-9.25 |
-4.50 |
+4.25 |
Overall
Lens Diameter (mm) |
8.1 |
8.6 |
7.9 |
8.2 |
8.8 |
8.5 |
7.9 |
| Optical
Zone (mm) |
6.7 |
7.2 |
6.7 |
6.8 |
7.3 |
7.1 |
6.7 |
| Peripheral
Curve (D) |
42.00 |
39.50 |
44.00 |
41.50 |
38.50 |
40.00 |
44.50 |
| Center
Thickness (mm)** |
0.10 |
0.14 |
0.27
|
0.21 |
0.10
|
0.11 |
0.27 |