I don’t naturally have it so visual explains don’t make sense to me
In: 6
Hold a finger up at arm’s length in front of you. Close one eye, and then the other, and notice how the finger appears to move against the background?
This effect is called *Parallax* and is fundamental to how we perceive depth. Because our eyes are not in exactly the same place, they see slightly different images of the world.
For depth perception, our brains take those two images and compare them. Over time, we’ve learnt to recognise that the closer an item is, the greater the difference in its apparent position to each eye, and thus approximate the depth of an item.
You can also ask a friend to focus looking at the tip of your finger, and watch their eyes as you move it closer and further away from them. You’ll be able to see their pupils move closer together as your finger gets closer to them. This is much the same idea: when looking at an object, if our eyes need to be closer together to merge the images then the object is closer to us.
Naturally, this all requires two eyes. There *is* a third factor that only needs one, involving recognising that closer items appear larger (cue Father Ted: [these ones are small, those are far away](https://youtu.be/MMiKyfd6hA0)), but this relies heavily on recognising shapes as physical objects with a known size. This is still enough to trip people up if something appears to be an object much larger or smaller than it really is.
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