Spectacle blur decreases stereoacuity more than ordinary visual acuity, and this cannot be accounted for by contrast reduction or by accommodation and/or convergence instability. Stereoscopic acuity was measured over a range of defocus exceeding 2 D in normal subjects with a 3-mm artificial pupil. Blur of one eye leads to a decrement in stereoacuity that is at least as high and usually higher than blur of both eyes; blurring only the disparate element of the pattern while keeping the rest clear is nearly as detrimental as blurring the whole pattern. In a separate experiment, the individual line elements of the pattern were subjected to spatial frequency filtering in the horizontal direction. For best stereoacuity values, representation of the whole spatial frequency spectrum is required. Patterns with diminished high-frequency content (> 10 cycles/deg) show lowered stereoacuity and those utilizing predominantly a spatial frequency band centered on 22 cycles/deg even more so.
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