A patient has an on-K GP lens. The GP trial power is -3.00 D, but the patient's prescription is -6.00 D. What is the over-refraction after vertex conversion?

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Multiple Choice

A patient has an on-K GP lens. The GP trial power is -3.00 D, but the patient's prescription is -6.00 D. What is the over-refraction after vertex conversion?

Explanation:
Vertex distance changes how spectacle power translates to a lens that sits on the eye. A spectacle correction of -6.00 D, when moved to a near-eye contact lens, is roughly reduced to about -5.50 D due to vertex conversion. The GP trial you have on the eye is -3.00 D. To reach the target of about -5.50 D, you need a further on-eye minus power of -2.50 D (since -3.00 plus -2.50 equals -5.50). So the over-refraction after vertex conversion is -2.50 D.

Vertex distance changes how spectacle power translates to a lens that sits on the eye. A spectacle correction of -6.00 D, when moved to a near-eye contact lens, is roughly reduced to about -5.50 D due to vertex conversion.

The GP trial you have on the eye is -3.00 D. To reach the target of about -5.50 D, you need a further on-eye minus power of -2.50 D (since -3.00 plus -2.50 equals -5.50).

So the over-refraction after vertex conversion is -2.50 D.

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