Monocular Cue: Relative Size

Unlike binocular cues, monocular cues are depth cues available to either eye alone. When we are judging how far away something is, binocular cues do not really help us. Monocular cues, on the other hand, can help us to determine how far away something actually is.

An important monocular cue is relative size. If two objects are fairly similar in size, our eyes perceive that the larger image is closer to us than the other image. Our brain interprets something smaller as something further away, not as a tiny object.

We have all experienced this, as we have stood on top of a skyscraper. Looking down from the top of the building, we can see people below. However, we know that these people are just further away from us, not that they are tinier people than we generally see on a daily basis. This is because of relative size, as we know that objects larger are generally closer to us than the objects that seem smaller.

In the two images below, the foot is the same size, but we are further away from the foot in the second example. So the foot looks smaller, but really it is the same size. It is just our distance from the foot that has increased.