It is generally believed that the ratio of accommodative convergence to accommodation (AC/A ratio) is relatively constant with age. This is based on studies of what is known as the AC/A stimulus ratio in which the denominator is the accommodative stimulus value. However, if the objective accommodative response to each stimulus is used instead in the denominator, then the ratio, called the AC/A response, is not constant but shows an increase beginning just prior to presbyopia. This suggests that greater effort is necessary for a unit of lens accommodation when one gets older or conversely a given effort produces less and less accommodation with age. The disparity between AC/A stimulus and AC/A response is discussed with reference to clinical presbyopia.
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