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John I. Yellott; Correcting spurious resolution. Journal of Vision 2005;5(12):97. doi: 10.1167/5.12.97.
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Spurious resolution is a byproduct of optical defocus whereby sinusoidal image components are reversed in sign: light stripes become dark and vice-versa. Such contrast-reversal effects are readily apparent in defocused projected images, and in subjective visual experience while viewing targets closer than the near point of the eye. Mathematically, spurious resolution is due to negative zones in the optical transfer function. Such zones appear in the transfer functions that correspond to blurring by convolution with a uniform disk (as in geometrical optics), and in those implied by physical optics for a defocused eye limited only by diffraction. What happens if these spurious negative zones are re-inverted, restoring the original contrast signs? The answer for both geometrical and physical optics is that the pointspread function is greatly re-sharpened. Since sign correction is purely a phase operation, this means that substantial deblurring can be produced by a simple phase-correcting filter that entails no amplitude restoration at all. In principle such a filter could be inserted at any point between a target (e.g., printed text) and its retinal image, by convolving the image at that stage (e.g., on the page) with the inverse Fourier transform of the sign of the blur transfer function.
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