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2/22/96 Chapter 6 Vision Assessment page 6-7
(1) Table 6-1 shows JND powers based on best corrected visual acuity from
retinoscopy.
(2) The values for the JND in Table 6-1 are suggested starting points. If the
patient responds quickly and/or easily to the starting JND power, then use the
next lower JND power. If the patient does not respond to the JND power, then
use the next higher JND power in routine subjective refraction as described
above in Section 6.2.
(3) Example:
Retinoscopy –4.00 = –4.00 X 130 20/400
JND power (from Table 6-1)
±2.00 D
Patient responds easily to ±2.00 D choices. Use the next lower JND power ±1.00
D.
(4) The cylinder refraction should be conducted using a hand-held Jackson cross
cylinder (JCC) of a magnitude approximately equal to the JND value. If the
acuity is between 20/50 and 20/100, use ±0.50 D JCC; if it is between 20/120
and 20/160, use a ±0.75 JCC; and if it is worse than 20/160, use a ±1.00 D JCC.
Proceed with the cylinder refraction as you would with a normally sighted
patient. If a significant change of cylinder power and/or axis occurs, recheck
the sphere. Remember to maintain the equivalent sphere when changing
cylinder power.
Note: Occasionally, it is useful for some patients to determine the best axis subjectively
by allowing them to manually turn the axis of the cylinder in the phoropter or trial
frame. Visual acuities need to be monitored with any changes in lens power to insure
better performance and to act as a guide to the proper amount of power for the JND
spheres and cylinders.
Table 6-1. Acuity after retinoscopy and power of the just noticeable difference lens
for subjective refraction.
Visual acuity after retinoscopy
JND power
20/20 to 20/40 ±0.25 D
20/40 to 20/100 ±0.50 D
20/100 to 20/200 ±1.00 D
20/200 to 20/400 ±2.00
Worse than 20/400
±3.00 D
6.4 Measurement of Over-Refraction