Static retinoscopy is performed with a plane mirror at a working distance of 67 cm. Neutralisation of the right eye is achieved with a -3.50 DS lens in the trial frame. Allowing for the working distance, the patient's actual refractive error is approximately:

• A -3.50 DS
• B -2.00 DS ✓
• C -5.00 DS
• D -1.75 DS

Explanation

The trial lens that neutralises the reflex includes the convergence needed for the observer's own position. At 67 cm the dioptric value of the working distance is about +1.50 D (100/67), and this must be algebraically subtracted from the neutralising lens: -3.50 plus 1.50 equals -2.00 DS. Failing to deduct the working distance leaves the error overcorrected by 1.50 D, which is exactly the trap in option A.

Reference: Parsons' Diseases of the Eye, 23rd ed.

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