OCULAR DEFOCUS, SPURIOUS RESOLUTION AND CONTRAST REVERSAL *

Abstract With the increasing popularity of periodic gratings in vision research, the effects of ocular defocus, such as spurious resolution and the presence of more than one maximum in the contrast–defocus level function, are very important. Optical transfer function theory is used to develop graphs and formulae that can be used to predict the presence of spurious resolution for given levels of defocus and spatial frequency in any aberration‐free optical system. Geometrical optical approximations are used extensively, as these lead to very simple formulae, These general equations, applying to any optical system, are extended to the particular case of ocular defocus. Formulae are presented that can be used 10 predict the presence of spurious resolution, in terms of such variables as pupil size and longitudinal focus error. Conditions for the observation of spuriously resolved gratings are also discussed. An example is presented for observing spurious resolution and contrast reversal and this example can be presented as a “pseudo” visual illusion.

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