HFA3 Instructions for Use 2660021166131 Rev. A 2018-11
Go to Contents Determine Trial Lens
B-1
(B) Determine Trial Lens
If you are not using the automatic trial lens calculation refer to the following guidelines for selecting
manual trial lenses.
Guidelines for Trial Lens Selection
1. Ignore cylinders of 0.25 D or less.
2. For cylinder errors of up to 1.75 D use the spherical equivalen
t. Use the full cylinder correction
for cylinder errors of 1.75 D or more.
3. Refer to Table B-1 for hyperopic or emmetropic patients, or Table B-2 for myopic patients, to
determine the power of the spherical trial lens needed.
4. Verify that the patient can see the fixation light clearly thr
ough the trial lens before beginning
testing. Young myopes may need additional minus power if the target appears blurry to them,
prior to beginning testing. Many 30–40 year olds may not need the full trial lens correction.
How to Calculate the Spherical Equivalent
The spherical equivalent is equal to half of the power of the cylinder (DC) correction. For example,
the spherical equivalent of +1.00 DC is +0.50 D. The spherical equivalent of –0.50 DC is –0.25 D.
Add the spherical equivalent of the cylinder power to the original spherical power to get the overall
spherical equivalent.
Age Hyperopic Distance R
x
is Greater than Zero Emmetropic Distance R
x
is Zero (Plano)
Under 30 Distance R
x
only No correction
30 –39 (Dist. R
x
) + (1.00 D) = trial lens +1.00 D trial lens
40–44 (Dist. R
x
) + (1.50 D) = trial lens +1.50 D trial lens
45–49 (Dist. R
x
) + (2.00 D) = trial lens +2.00 D trial lens
50–54 (Dist. R
x
) + (2.50 D) = trial lens +2.50 D trial lens
55–59 (Dist. R
x
) + (3.00 D) = trial lens +3.00 D trial lens
60 & Over (Dist. R
x
) + (3.25 D) = trial lens +3.25 D trial lens
Table B-1 Spherical Trial Lens Correction for Central Visual Field Testing of Hyperopic and
E
mmetropic Patients
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