Rules, the old saying goes, are made to be broken, and rules about what makes a good photograph are no exception. A page on the National Geographic website offering tips to improving your photographs argues that "It’s fine to know the rules, but it’s also essential to break them. Strictly adhering to the rules may ensure mediocre photographs. Top photographers break the rules as often as they keep them."

One of the basic rules of stereo photography is avoid retinal rivalry. Retinal rivalry, according to an on-line medical paper, is the effect produced "when two dissimilar images fall on the corresponding areas of the retina…there is confusion due to their superimposition and a conflict between the two retinas. The brain therefore suppresses one of the images. If both eyes have equal dominance there is alternating suppression of these images." Most people experience this as a sort of annoying flicker.

Nancy and Landon’s Hearth, chosen one of the OCC Best Five (B Group) Slides of 2002 despite its firey ‘retinal rivalry’

Retinal rivalry is easiest to produce when making a stereo pair with one single-lens camera taking two shots in succession. It is easy to overlook moving elements in what seems to be a static scene. All it takes is one rustling leaf in a corner or a distant car that’s there in only one shot to mar and even ruin an otherwise effective picture. The effect is so noticeable that the eye goes to it immediately. This bothers some people more than others, but usually there is good reason for the rule.

Other methods of producing stereo images are not immune from the dangers of retinal rivalry. If you have twinned cameras but the focal length settings are not exactly the same, or the lenses are significantly different, or if one lens has a filter on it but the other does not, everything you shoot will set the eyes to bickering. On older stereo cameras, the lenses can go out of alignment, and even if the camera is fine the photographer may not be--just stick the tip of your thumb in front of one of the lenses and there you go (of course, I’m sure no one reading this has ever done anything of the sort.) The disorienting effect produced when a shot is taken with a camera that isn’t quite level is a form of RR--the brain wants the parts of the image that seem to correspond to appear on corresponding parts of the retina, and if the tilt is too dramatic the whole image will set those retinas at war.

So why would one ever want to break this rule? Well, how about when the part of the image that seems to flicker is supposed to flicker? Such is the case with Nancy and Landon’s Hearth, an image of an open woodstove shot with a single Nikon SLR mounted on a tripod with a slide bar. Everything but the fire is in normal, unmoving stereo. The fire flickers, but since the brain expects fires to flicker it doesn’t find the retinal rivalry a problem. In fact the picture is enhanced by the induced sense of dynamism. At least that’s what the judges seemed to think.

Deliberate use of retinal rivalry is infrequent but effective. I’ve seen it used for dramatic effect in a comic book by Ray Zone, Battle for a Three Dimensional World, and to suggest the motion of a juggler’s hands in a charming book of stereo drawings.

It’s not the easiest technique to use effectively, however. Since my success with Hearth I’ve tried out a couple others on OCC judges, including a picture of a stretch of coastline in which retinal rivalry was deliberately used to suggest the dynamism of a thin line of surf. Everyone hated it.