Retinal Disparity (Binocular Cue)

Human eyes receive slightly different images because of the distance apart they are. The brain compares the two images, and the difference is called Retinal Disparity. It's an important cue to the relative distance of different objects.

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When you hold your two index fingers in front of your eyes, like the boy, you should see a floating "finger sausage". This is called the retinal disparity. It comes from your two retinas seeing different things.