According to Knapp's law, aniseikonia in purely AXIAL anisometropia is minimised when the correction is given by:

• A A spectacle lens positioned at the posterior focal point of the eye
• B A contact lens on the corneal surface
• C A spectacle lens positioned at the anterior focal point of the eye ✓
• D An intraocular lens in the capsular bag

Explanation

Knapp's law states that if a correcting spectacle lens is placed at the eye's anterior focal point, the retinal image size in an axially ametropic eye equals that of the normal fellow eye, because the changed vergence compensates exactly for the changed axial length. Hence axial anisometropia theoretically favours spectacles. Refractive anisometropia, such as unilateral aphakia, behaves oppositely and is far better served by a contact lens, which is why option B kills itself in the axial setting.

Reference: Elkington AR, Clinical Optics, 3rd ed.

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Written and medically reviewed by the StethoPrep medical team.