In the calculated AC/A ratio, what unit is used for near fixation distance?

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Multiple Choice

In the calculated AC/A ratio, what unit is used for near fixation distance?

Explanation:
AC/A ratio ties how much convergence (in prism diopters) occurs to how much accommodation (in diopters) is used. To get the accommodation demand at near, you convert distance to diopters using the relation D = 1 / distance (in meters). So the distance must be in meters because diopters are the reciprocal of meters. For example, at 0.40 m, the near accommodative demand is 1 / 0.40 = 2.50 D. This makes meters the correct unit for near fixation distance in the calculated AC/A ratio.

AC/A ratio ties how much convergence (in prism diopters) occurs to how much accommodation (in diopters) is used. To get the accommodation demand at near, you convert distance to diopters using the relation D = 1 / distance (in meters). So the distance must be in meters because diopters are the reciprocal of meters. For example, at 0.40 m, the near accommodative demand is 1 / 0.40 = 2.50 D. This makes meters the correct unit for near fixation distance in the calculated AC/A ratio.

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