Notes by Dr. Optoglass: Stereoscopy – Understanding Depth

Topics Covered:

  • Depth and Depth Cues
  • Monocular and Binocular Vision

And so if your inclination is to champion lost causes, the case of stereo photography is readymade for you – Paul Farber

So far we have treated the eye as if

  • It were an organ capable of only two-dimensional (flat surface) vision
  • We had only one of them

When we take only one eye into consideration we are said to be referring to monocular (one eye) vision.

We live in a three dimensional world – we can move in the left-right, forward-back and up-down directions. What this has given us, is an impression of depth.

Depth is a perception – a feeling of space. If we only had monocular vision, how does the mind-eye combination create this feeling? It does this with small clues that we take for granted, but use subconsciously to perceive depth.

These are called Monocular Depth Cues, some of which are as follows:

  • Occlusion
  • Motion parallax
  • Depth from motion
  • Perspective
  • Relative size
  • Familiar size
  • Aerial perspective
  • Accommodation
  • Curvilinear perspective
  • Texture gradient
  • Lighting and shading
  • Defocus blur

When one object hides another, it is said to be occluding the other.

We know the bird is closer to us because it is blocking portions of the wall.

Motion parallax
Ever looked out of a train while it was in motion? The foreground appears to move faster, while the horizon barely appears to move. This is motion parallax in action.

Depth from motion
Objects moving away reduce in size. Those moving closer increase in size. The rate of this change tells the brain how fast an object is travelling towards or away from us, and how far it is.

The property of parallel lines converging at infinity is perspective. We are always looking for some perspective, and our perspective gives us our path.

Relative size
If two objects are known to be the same size, relative size cues can provide information about the relative depth of the two objects.

Familiar size
If we already know from memory (or frequent familiarity) the size of a particular object, we can use that information to determine the absolute depth of the object.

Aerial perspective
Due to light scattering by the atmosphere, objects further away have lower contrast and saturation. Sometimes far away objects also have a bluish tone.

We have already seen that our needs to exert itself to focus. When it does this, it also sends a signal to the brain – from which our mind draws conclusions about the depth of the object we are focusing upon.

Curvilinear perspective
At the outer extremes of the visual field, parallel lines become curved – this distortion gives a feeling to our sense of depth just as linear perspective does. It’s our unique interpretation of the world.

Texture gradient
Texture is perceived differently depending on its distance to us.

Lighting and shading
The way in which light falls on objects, creates shadows and highlights, tells us a great deal about the depth of a particular scene.

Defocus blur
When focusing on a close object, the background automatically blurs – try it by focusing on your finger.

All these properties contribute to adding depth to our experience of the world. Can we do better?

Yes – one of the coolest things about our eyes is that we have two of them. What does the second eye give us?

Binocular Vision

If we use both eyes to see, we are said to be using binocular vision.

Two eyes give us double the information. It’s not the same thing because the eyes are not in the same place!

The clues provided by binocular vision are called binocular depth cues. In the following pages I’ll outline the properties of binocular depth cues.


  • Vision through one eye is monocular vision. When both eyes are used the vision is called binocular vision.
  • The brain uses both monocular and binocular depth cues to create a sense of space.
  • Binocular vision offers us two perspectives of the same scene at the same time.

Links for further study:

Next: Stereopsis, Parallax and the Interocular Distance
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