The blur point during smooth vergences represents the limit of which vergence?

• A. Limit of fusional vergence only
• B. Limit of accommodative vergence only
• C. Limit of accommodative and fusional vergence
• D. Limit of proximal vergence only

Answer: (A)

The main idea is that the blur point in smooth vergence testing reveals how far the eyes’ fusion system can compensate to keep the image single. In this test, prisms gradually change the vergence demand while the target distance stays fixed, so accommodation stays relatively constant and proximal cues don’t drive the response. The only system being pushed to its limit is fusional vergence—the fine-tuning of eye alignment to maintain single vision. When fusional vergence reserves are exhausted, fusion can no longer be maintained, and the image appears blurred. Accommodative vergence isn’t the limiting factor here because accommodation is not being dramatically taxed by the test, and proximal vergence isn’t the focus of this particular measurement.