Stereopsis is a critical component to human vision and is a visual trait shared by animals with front-facing eyes. The spacing between our eyes gives us an advantage. This slight offset causes our visual system to see an image from a slightly different perspective (try it now by looking at something far away and moving your hand back and forth between each eye!) The combination of these slightly different images allows the visual system to judge fine depth and create a 3-dimensional interpretation of the image. This becomes important when objects are close to one another or the task requires the visual system to provide exact information on where the body is relative to an object (thing threading a needle).
The slightly different perspective each eye sees is due to images falling on slightly different points on the retina of the eye. This is termed disparity and is the critical item that is measured in a stereopsis test.
Stereopsis is measured in the value seconds of arc - this is actually a very small value. To understand how small, first think of a circle that is made of up 360 degrees all the way around - the other way to think of this is a circle is made of 360 individual 1-degree segments. So, one degree of a circle represents 1/360th of the circle. In geometry, this 1-degree segment can be further divided into 60 smaller segments, termed minutes of arc. Each minute of arc can be further divided into 60 smaller segments - this is termed a second of arc (or arcsecond). So one arcsecond is 1/3,600th of a degree or 1/1,260,000th of a circle! That's some really fine vision!
Stereopsis is divided into two main components - course (or gross) stereopsis, which is important for large amounts of depth, and fine stereopsis, which is important for very small amounts of depth. Most stereo tests check both.
There are different methods of stereo testing targets. Random dot targets at first glance look like a bunch of scattered dots. These targets require the patient to combine the images to see a shape or pattern. Countour targets have a distinct shape but use smaller and smaller offsets of the shape to measure stereo ability. Most tests require some form or dissociation, either with polarized lenses or with red/green or red/blue glasses.