Interaxial

The interaxial (stereo base) is the distance between the optical centers of the two lenses. It is the main variable that determines the amount of depth.

It should not be confused with the viewer's interocular distance (IPD). The interocular distance averages around 63 mm in adults. The interaxial, by contrast, can range from a few millimeters to meters depending on the composition.

Depth range

The parallax difference between the nearest and the farthest point of the scene is proportional to the interaxial:

where: R depth range (ratio of image width), f focal length, Wc sensor width, Zn and Zf the nearest and farthest point of the scene

Example: 35 mm lens, Super 35 sensor, scene between 2 m and 20 m. With a 65 mm interaxial the range is 4.1% of the image width; with 45 mm, 2.8%.

Roundness

The ratio of an object's perceived depth on the screen to its perceived width is measured by roundness (roundness factor; shape ratio in Spottiswoode). Near the screen plane:

where: ρ roundness, V viewing distance, C convergence distance, e interocular distance

• ρ = 1: the object appears with its natural proportions. The condition is b / C = e / V (depth consistency).
• ρ < 1: the object flattens. Below 0.2, cardboarding becomes noticeable (Mendiburu).
• ρ > 1: the object stretches in depth.

Roundness depends on the viewing distance, not on the screen width. For example, with a 65 mm interaxial, the main subject at 3.5 m and a TV viewed from 2.5 m, ρ ≈ 0.71.

Divergence limit

If the screen parallax of a point at infinity exceeds the interocular distance, the eyes are forced to turn outward. The largest interaxial that prevents this is:

where: M frame magnification: the ratio of screen width to sensor width

For the same shot, this limit tightens as the screen grows. With a 35 mm lens, a Super 35 sensor and 3.5 m convergence: 133 mm on a 55" TV, 16 mm on a 10 m screen.

Hyperstereo and hypostereo

If the interaxial is much larger than the interocular distance (hyperstereo), the scene can be perceived as shrunk, like a scale model. If it is much smaller (hypostereo), the scene can conversely be perceived as enlarged. This is why a large interaxial is common for distant landscapes and a small interaxial for macro shooting.