addition, near (add)
The difference in spherical power between the distance and near corrections. A common method of arriving at the power of the addition is to measure the patient's working distance and the amplitude of accommodation. The add is obtained as follows
add = (1 metre/working distance in metre) − x(amplitude)
where x is the percentage of the total amplitude of accommodation which is to be used: two-thirds is usually more appropriate for young presbyopes (below about 52 years of age), while one-half is more appropriate for older presbyopes. Thus, this formula allows for a certain amount of the amplitude of accommodation to be left in reserve (usually one-third or one-half). It can also be estimated based on age: +1.0 D to +1.25 D for age 45-50 years; +1.5 to +1.75 D for age 50-55 years; +2.0 D to +2.25 D for age 55-60 years; +2.5 D to +3.0 D for older patients. However these figures may need to be adjusted to take into account the average working distance of the patient. Syn. reading addition. See bracketing; reading distance; presbyopia; cross-cylinder test at near.)
Fig. A6 Abduction of the right eye. Adduction of the left eye
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann