Differences in the nearpoint of convergence with target type

The purpose of this study was to determine the effect on measurements of the nearpoint of convergence (NPC) of different target types. In order to assess the influence of accommodation, the NPC was also measured under conditions of varying accommodative demand.

The NPC was measured to the nearest 0.5 cm using three targets: the RAF rule, the sharpened tip of a pencil and the tip of the examiner's index finger. All measurements were performed under the same conditions on two groups of asymptomatic subjects, a group of 14 presbyopic subjects and a group of 14, younger, non-presbyopic subjects. The influence of accommodative demand was assessed in the non-presbyopic group by measuring the NPC while subjects viewed the RAF rule target through +2.00 and -2.00 lenses held in front of their eyes.

For the presbyopic group, the NPC (break) and NPC (recovery) were independent of target type. However, the NPC (break) was significantly less remote than the NPC (recovery). Comparative data for the non-presbyopic group showed that NPC (break) for the RAF target was less remote than for either the pencil tip or finger tip targets. In agreement with the results from the presbyopic group, the NPC (recovery) was independent of target type.

For subjects with little or no accommodation, the NPC does not depend on the target used and is the same measured with the RAF rule, a pencil tip or finger tip. In non-presbyopic subjects there appears to be a small accommodative influence on the NPC, which is target dependent. However, the difference is probably not clinically important.