Image-sharpness criteria studied previously have depended on the assumption of the spatial stationarity of the point-spread function of the degrading medium plus the imaging lens system. If this assumption does not hold, the sharpness criteria no longer necessarily reach their extrema when system aberrations are zero. A criterion has been constructed that avoids this dependency, although at the expense of requiring information that is unnecessary in the spatially stationary case. After the instantaneous degraded image is filtered with a transfer function that is equal to or slightly greater than the transfer function of the degrading medium, the resulting image must be equal to the long-term-averaged image. The instantaneous image before modification is therefore diffraction limited or almost diffraction limited. The criterion may be applied locally.
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