Abstract: An understanding is required of the range of image quality reduction that can be tolerated by human subjects. The modulation transfer function (MTF) of the phakic eye provides a reasonable reference for nominal optical performance. We propose MTFs of a 1/4D and 1/2D defocused eye as minimum acceptability criteria. Weber's law can be used to assess whether differences from nominal values are likely to be detected. These criteria apply to assessment of distant object imaging. To predict levels of clinical measures based on in-vitro MTFs, a simple model of human threshold recognition/detection is discussed. This Expected Visual Outcome model provides a tool for estimating the clinical efficacy of a particular optical surface design. !17
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