anishing point at the horizon. The more the lines converge, the farther away they appear.5) SIZE CLUESAnother visual cue to apparent depth is closely related to size constancy. According to size constancy, even though the size of the retinal image may change as an object moves closer to us or farther from us, we perceive that object as staying about the same size. We are able to do so because we take distance into consideration. Thus, if we assume that two objects are the same size, we perceive the object that casts a smaller retinal image as farther away than the object that casts a larger retinal image. This depth cue is known as relative size, because we consider the size of an object's retinal image relative to other objects when estimating its distance.Another depth cue involves the familiar size of objects. Through experience, we become familiar with the standard size of certain objects, such as houses, cars, airplanes, people, animals, books, and chairs. Knowing the size of these objects helps us judge our distance from them and from objects around them.6) HEIGHT CLUESWe perceive points nearer to the horizon as more distant than points that are farther away from the horizon. This means that below the horizon, objects higher in the visual field appear farther away than those that are lower. Above the horizon, objects lower in the visual field appear farther away than those that are higher. For example, in the accompanying picture entitled "Relative Height," the animals higher in the photo appear farther away than the animals lower in the photo. But above the horizon, the clouds lower in the photo appear farther away than the clouds higher in the photo. This depth cue is called relative elevation or relative height, because when judging an object's distance, we consider its height in our visual field relative to other objects.7) MOTION PARALLAXThe monocular cues discussed so far—interposition, atmospheric perspective, texture...
