zone of clear, single, binocular vision
In Donders' diagram it is the region determined by the extremes of accommodation and convergence that can be evoked while retaining a clear, single image. Clinically, this is determined by measuring the limits of negative and positive relative convergence by using base-in and base-out prisms to blur, or by measuring relative accommodation by binocularly adding concave or convex lenses, for various binocularly fixated distances (Fig. Z1). See relative amplitude of accommodation; relative convergence; Donders' diagram; rotary prism; binocular vision.
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Fig. Z1 Zone of clear, single, binocular vision (NRC, negative relative convergence; PRC, positive relative convergence; NRA, negative relative accommodation; PRA, positive relative accommodation; BI, base-in prism; BO, base-out prism)
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann