Depth perception is the ability to see objects in three dimensions and estimate their distance from the observer. It is a critical part of everyday life, allowing us to judge distances when driving, catch a ball, or simply reach for an object.
There are two main types of depth perception: monocular and binocular. Monocular depth cues do not require both eyes to work; they can be perceived with one eye alone. Examples include relative size, interposition, linear perspective, and texture gradient. Binocular depth cues, on the other hand, involve information from both eyes working together. This type of depth cue is sometimes called stereopsis. Important binocular cues include convergence (the movement of the eyes inward), retinal disparity (the difference in image position between the two eyes), and accommodation (the changing focus of the eye).
The brain uses these various cues to construct a three-dimensional representation of the world around us. Depth perception is not perfect, however; there are many ways in which it can fail or be fooled. For example, our brains may incorrectly interpret linear perspective cues, leading us to misjudge distances (this is why parallel lines appear to converge at a distant horizon). Similarly, our brains may use shadows as an indication of an object’s position even when there is no light source present (this can happen when we look at pictures that have been taken with a flash). Other times we may misjudge distances because we are relying on one type of cue too heavily – for example, if we are looking at an object through a windowpane we may underestimate its distance because we are only seeing it through one plane surface.
Despite these potential errors, depth perception is generally quite accurate and allows us to navigate our environment successfully on a day-to-day basis.