ABSTRACT: The tilting of the front-parallel plane with a monocular reduction of retinal luminance is considered. The reduction in retinal image size with lower illumination is considered as an explanation with the inconsistencies in the evidence both in frontal plane judgments and observations of the Pulfrich effect. Experimental work shows a tilt of the frontal plane consistent with a shift in the binoculus towards the eye with the higher illumination. This movement of the binoculus is offered as a second factor affecting the frontal plane judgments with unequal retinal luminance.