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  The Contrast Sensitivity Function (CSF) shows that visual sensitivity to contrast is highest at lower resolutions than the threshold of acuity (the point where we can no longer see contrast difference). This is noted by the envelope of modulation that looks like a Gaussian curve, a symmetrical bell-shaped curve of normal distribution that encompasses the more-defined blacks. If the pattern is observed up close, the Gaussian shape moves right; as you view the pattern from greater distances, it will continue to move left. The peak on this curve represents the spatial frequency where contrast discrimination is most sensitive.