Increasing perspectival obliqueness increases the leaning tower illusion

The leaning tower illusion is a perceptual illusion in which two identical images of a tower photographed from below appear to diverge when juxtaposed. We manipulated the perceived obliqueness of the (upright) St Mark bell tower in Venice by modifying two parameters both related to the position of the camera with respect to the tower: (a) increasing the peripherality of the tower and (b) reducing the distance between the camera and the tower. The resulting images clearly show that the illusory leaning effect increases as a function of the obliqueness. Another crucial condition for the leaning tower effect must be that the twin images are perceived as parts of a unitary display: The illusion increases when the distance between the photos is progressively increased, but beyond a certain level of separation, the integration of the images should, of course, break down, and the illusion vanish.