Notes by Dr. Optoglass: Stereoscopy – Convergence and Fusion

Topics Covered:

  • Convergence and Fusion
  • The Singleness of Vision

Taking aim for too long can ruin your eyes – Ivorian Proverb


When we focus on infinity our eyes look straight on in a parallel fashion, as indicated by the red arrows. As we have already seen, optical tests on the Snellen chart are positioned at 20 feet so that the eye is focusing on infinity.

What happens when the object is closer than, say, 15 feet? The eyes draw inwards as indicated by the blue line. This is called convergence.

There is a ‘sensor’ in our brain capable of knowing when it is getting only one image (one eye closed) and when it is getting two images. When it gets two images, it looks for the object the eye is focusing on. In the above example, it is the bird.

Since two eyes have two perspectives, shouldn’t we see two images? We don’t, because the ‘sensor’ in our brain fuses both the images together to give us a perception that is called the singleness of vision. This process directly relates to our feeling of depth in a scene.

Is convergence a fixed property like the inter-ocular distance? No, as can be clearly seen from the above image. Depending on the distance and angle of the object, the eye converges differently. Also, not everyone’s eyes are perfect symmetrical lenses.

To make matters even more complicated, the eye’s ability to converge fights its ability to accommodate – yet it does it so brilliantly without our being aware of the fact. One fixed outcome of this is that the eye always converges on what it focuses.

E.g., if you’re looking at a bird, the eye focuses on the bird and converges to it as well. Always. It’s like a Kung-fu movie where two deadly foes (convergence and accommodation) battle each other in step – which visually results in an elaborate and beautiful dance.

Next, we will look at how these factors impact the camera’s ability to reproduce the stereoscopic effect that the eye does so seamlessly.


  • Convergence is the eye’s ability to physically lock on an object closer than the ‘infinity’ focus distance.
  • Fusion is the brain’s ability to seamlessly merge two image streams from both eyes into one singleness of vision.
  • The eye always converges on the object it focuses at.


Next: Challenges in Reproducing Stereopsis in Cameras and Displays
Previous: Stereopsis, Parallax and the Inter-ocular Distance