We have developed a new set of tests for evaluation of visual function through media opacities, based on vernier acuity measurements (hyperacuity). In this paper, results of one of these tests, the 'gap test', are compared in patients with posterior subcapsular (PSC) cataract versus nuclear cataract (NC). Patients with PSC cataract often report multiple images or significant 'star burst' effects. We hypothesized the presence of 'multi-prismatic' and/or high frequency spurious resolution phenomena due to PSC cataract characteristic substructure. We were able to minimize these effects by using 1) a pinhole held close to the eye, 2) a large (adapting) background field of white light superimposed on the vernier test targets, 3) a low-pass spatial filter applied to the targets. When the particular problems associated with PSC cataract are not present or are adequately addressed and when patients are matched for visual acuity, the hyperacuity 'gap test' shows less functional effect due to the opacity for PSC cataract than NC in all the cases we have tested. These findings, moreover, indicate that visual acuity provides an insufficient description of the effects of intraocular scattering on image formation. The results emphasize the importance and the necessity of developing models that better clarify the specific effects ol different types of ocular media opacities.