Most people see naturally through two eyes. The eyes are placed an average of 63.5mm apart - depending on genetic factors. Two similar but different viewpoints generate a small amount of parallax error where the depth of the scene viewed is not equivalent to the point the eyes are converged at. The computational power of the brain then takes over and facilitates depth perception. There is a subgroup of the population who can only see out of one eye due to anatomical or physiological problems and cannot see stereoscopically.
Binocular parallax is a powerful depth cue in binocular individuals, but it is by no means the only one used by the mind. A lot of depth cues are available to one eye and anything that can be seen as a depth cue in a standard photograph can be included in this category.
Motion parallax is a depth cue which can be used by the monocular individual or the cinematographer: this occurs due to a smaller amount of motion in a scene when an object is far away - for example in scenes from a moving train window. Perspective and texture gradient allow depth perception by objects getting smaller as the distance from the user increases. Fog and haze increasing with distance also contribute to depth perception. Occlusion of objects by other objects which are nearer to the viewer are powerful depth cues that are available in normal scenes but are not applicable to radiographs. Known objects also help with depth perception - where a object of known size (say, an elephant) appears in the scene, one can often make a guess as to how far away it is and compare it to other objects in the same scene. Shading and lighting also plays an important role, especially if one knows where the light source is or if it is included in the scene.
Benefits of stereoscopy
A well presented stereoscopic image is pleasing to look at. The depth of structures is readily apparent and the appreciation of details not observed before becomes evident. The combination of two views can provide more useful information in a scene than a single view, or two views that are taken from widely disparate viewpoints. Relative depth can be easily gauged and with proper measurement apparatus (as in aerial photography or radiostereogrammetrical analysis) absolute depth can be measured in a pair of images.
Problems in stereoscopy
A poorly presented stereoscopic image leaves no impression; or one of headaches, eyestrain and nausea with the unfortunate viewer.
Most of the bad stereoscopic experiences (of which there are many) are due to poor presentation. It is not uncommon to see images that are presented with rotation or vertical parallax errors, and also not uncommon to see images with the views flipped (pseudostereo). Geometry is also important and if this is changed (eg. shifting from small screen to large projection screen), eyestrain may be expected.
Accommodation-convergence mismatch is the problem that arises when the projected image plane is in focus but the eyes are converging at a different depth plane. Humans have a tolerance for some mismatch but when this becomes too large, either the image defocusses or stereo fusion is lost. This link between accommodation and convergence is hard-wired into our nervous systems but can be overcome to an extent by practice. It is also easier to compensate for viewing close images (converging) as most people are not able to make their eyes diverge.
Ghosting is the presence of a small proportion of the image intended for the other eye being visible. It is a problem for most of the computer based viewing systems where the image pathways are not kept physically separated. Where high contrast images are used (such as when looking at metallic implants on a radiograph) ghosting becomes a significant problem. The solution has yet to be devised for computer viewers, but using a purely optical viewing system should not give any ghosting.