Corneal edema [in man] was produced by atmospheric-induced anoxia and the visual effect was assessed by conventional acuity tests and compared with contrast sensitivity measurements. For the degree of edema which this technique produced, contrast sensitivity was depressed for only high spatial frequencies and could be considered in terms of equivalent defocus. This was not the case for a greater degree of edema which was simulated by a diffuser with a particle size that mimics the diffraction effects from an edematous cornea. Equivalent blur was not applicable, since low spatial frequencies were also affected and the letter and contrast-sensitivity tests gave radically different results. The validity of the present letter acuity evaluation of visual function for patients in which intraocular scattering is involved was questioned.