In compound hypermetropic astigmatism, where does the circle of least confusion lie relative to the retina?
- A On the retina itself
- B Anterior to the retina
- C Posterior to the retina, between the two focal lines ✓
- D Exactly at the posterior focal line
Correct answer: C. Posterior to the retina, between the two focal lines
Explanation
In compound hypermetropic astigmatism both principal meridians focus behind the retina, so every element of the Sturm conoid, including the circle of least confusion, falls posterior to the retina. The circle of least confusion always sits between the two focal lines, never at either one, which eliminates option D. It would fall on the retina only in simple hypermetropic astigmatism with one focal line on the retina, and anterior to the retina in any myopic astigmatic state.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
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