If a distant object is viewed through a small ciruclar aperture that is placed close to the eye, there is an apparent "shadowing" of the center of the viewed image, while the inner perimeter of the aperture is bright. The small aperture enhances the depth of field for good focus, but the dimming of the center of the image makes viewing more difficult. The aperture is defocused because of its proximity to the eye, and because the point of focus is intentionally at a distance. If a pair of short slits in the form of a cross (with width equal to the original circular aperture) is used instead of the circular aperture, the center of the aperture image is bright. The aperture size (~ 1.0mm) is large compared to the wavelength of light which would suggest that diffraction is not the cause of this phenomena. Defocus can account for the general dimming of the image, but not the brightness around the perimeter. The cross-slot aperture appears to solve the problem of dimming at the expense of depth of field. Can anyone explain what causes these effects?