Stereoscopic acuity

Taking into account that a small angle expressed in radians can be approximated by its tangent, the formula to calculate stereoacuity dγ is this: where a is the interocular separation of the observer, z the distance of the fixed peg from the eye and dz the position difference.

To transfer dγ into the usual unit of minutes of arc, a multiplicative constant c is inserted whose value is 3437.75 (1 radian in arcminutes).

These very small values of normal stereoacuity, expressed in differences of either object distances, or angle of disparity, makes it a hyperacuity.

Since the Howard-Dolman test described above is cumbersome, stereoacuity is usually measured using a stereogram in which separate panels are shown to each eye by superimposing them in a stereoscope using prisms or goggles with color or polarizing filters or alternating occlusion.

Optimum stereoacuity requires that the following mitigating factors be avoided: More than other such visual capabilities, the limits of stereopsis depend on the observer's familiarity with the situation.

[9] This is most vividly evident in the time it takes to "solve" a random-dot stereogram rapidly decreases between the first exposure and subsequent views[10]