Mark Changizi has been producing papers since 2008 around a theory of the function of human binocular vision. It is expressed in his The Vision Revolution in an engaging Chapter Two X-Ray Vision which, like the others commences under a Kipling quotation. Changizi’s self-assured,‘Just-So’, style may irritate some (does he sometimes overreach mere hypothesis and available evidence?), but his take on the use of two eyes is nevertheless novel and convincing; that the purpose of two eyes facing forward is to enable the owner to ‘see-through’ the clutter of enclosed jungle or tall grass (and defeat also the 2D subterfuge of camouflage referred to in the Kipling tale). By providing the maximum degree of overlap, adjustable via control over convergence, large parts of the scene are rendered transparent… hence “X-Ray vision”.*
In Changizi’s The Vision Revolution, the effect is illustrated with line drawings, but might photographs have delivered still other revelations of convergent vision in a complex environment?
In this regard may I suggest a photographic experiment? Make, with the basic mechanics of stereo imaging, two shots of a scene from two positions the same distance apart, or for this purpose, more widely-spaced than human eyes. You depart from the usual parallel setting of the lenses of the stereoscope camera. Employ the calibrated view camera ground-glass to position each view to overlap on the same point or object in the scene. These binary images are not to be set side by side to render 3D with special viewers, but are exposed on the same sheet. In other words these are convergent double images.
I exhibited (long ago now in 2001 at Bendigo Art Gallery) a series of monochrome mural images employing this technique. The double exposures were made, amongst other reasons, in order to confirm my own questions about the existence of an intermediate stage of vision that is probably impossible to perceive with the naked eye without a photographic analogy; given our front-facing eyes, at some point our visual processing must deal with just such incoming imagery, and it reveals some remarkable evidence for the way our attention works in such challenging environments.
Look at foreground and background in these images; there confusion and chaos is doubled. But at the points of convergence (more than one due to the horopter) the object of attention stands out at the still point of a vortex of spiralling repetitions. In accord with Changizi’s hypothesis, there is transparency certainly, but only where attention is less, while convergence (read concentration) renders an extraordinary clarity.
It is still worth asking, as Ernst Mach did in 1893, ‘Why do we have two eyes?’.
[* News stories on this include: Forbes, Le Point, Science Daily, Caltech Press Release, Times of India, Kotaku, Wissenschaft, Epoch Times, Spiegel, Membrana, Polska, Skeptically Speaking, Real Science (audio), a Scientific American interview, and in Rensselaer Magazine.]