In today’s post, we explain the two binocular cues that give us depth perception. We also discuss binocular vision and why it benefits us.
Binocular cues are the information taken in by our 2 eyes and then processed by our brain in order for us to perceive depth or distance.
These cues are vitally important to us, as without them we would have no idea how steep the path is that we’re climbing, or how far away an incoming ball is.
Binocular cues allow us to gain a 3-dimensional interpretation of the world and allow us to navigate through it so effortlessly.
Depth perception is otherwise known as stereopsis. It’s our ability to judge where an object lays in space relative to our own position. It allows us to easily distinguish between things that are near and far away.
Binocular cues are sensed with our eyes and this information is fed to our brain which processes the information for us to use. In this article, we’ll take a look at the two binocular depth cues. These are retinal disparity and binocular convergence.
Otherwise known as binocular parallax, retinal disparity addresses the fact that our two eyes each see different images. Our eyes are separated by an average distance of 6.3cm on our face, this means they each see life from a slightly different angle. This point is illustrated best if you hold your hand out in front of you and look at it through one eye at a time. You’ll notice that the image “shifts” slightly each time.
Our brain effectively triangulates the distance to an object, using the two different images that are eyes present to us. It seamlessly merges these two images into the picture that we see that contains the 3D information that is crucial to us. The larger the difference (disparity) in the 2 images, the closer an object is to you. A far away object contains little disparity in the 2 images seen by each eye. This time if you hold out your hand in front of you, but now start close to your face then slowly move it away. You’ll notice that the image seen through each eye will be more similar with increasing distance from you.
3D movies that you see at the theater are a great example of retinal disparity. The technology uses this binocular cue to great effect and tricks our brains into seeing a greater depth of field. The movie is filmed with 2 cameras each representing a slightly different view of the scene. We then wear special glasses that enable the left eye only to see the image from the left camera, and the right eye only the right camera.
Retinal Disparity Definition: the difference in the 2 images seen by each eye due to the difference in angle each is viewing an object from.
Binocular convergence is the other important cue that enables us to determine how near or far things are away from us. It refers to the amount of rotation your eyes have to do to focus on an object. Your eye muscles must contract and relax in order for you to focus on objects at different distances. The brain processes these muscle movements into information that is used for depth perception. Binocular convergence is in short, the amount of inward rotation your eyes have to do in order to focus on an object.
Binocular convergence is a proprioceptive sense (a sense that shows our position in space). It uses the information from the eye muscles (feedback) to gauge how much the eyes have rotated, and therefore how far an object is.
Like with retinal disparity, there’s a simple way of observing this binocular cue in action. If you hold a hand out in front of you at arm’s length, and then slowly bring it closer to your face. Eventually, your eyes will begin to strain and the image will blur and become 2 images. If done in reverse, starting close to your face and slowly moving away, you’ll notice the opposite happen. You start with 2 images and finish with just one.
Advantages of Binocular Vision
Binocular vision is the ability of an animal with two eyes to perceive life in 3 dimensions. There are two binocular cues which allow an animal or human to do this and we covered both of them in the previous sections. Having binocular vision is said to provide 6 main benefits. Manfred Fahle states these as being:
- You effectively have a spare eye. If one is lost or damaged you still have one left.
- It gives a wider field of view.
- We can use retinal disparity (see next section) to distinguish distance.
- We can also use binocular convergence (see next section) to distinguish distance.
- We can partially see an object behind an obstacle.
- It allows binocular summation. This improves contrast sensitivity, visual acuity, brightness perception, and flicker perception.
Binocular Cues in Nature
Many herbivores lack a detailed sense of depth perception as their lifestyle simply doesn’t require it. Open plain herbivores like cows have eyes on the sides of their heads (monocular vision). This gives them a huge field of vision, which is much greater than our own. This is perfect for spotting any would-be predators. However, it means they have a very poor judgment of distance.
Hunters like eagles and big cats have eyes on the front of their heads. This gives them depth perception which is crucial to a successful kill. They must know how far away their prey is and when is the optimal time to pounce.
Some animals that spend time in tree-tops also require good binocular vision. Monkeys are one such example. They require accurate information about distances to nearest branches, especially when moving at speed. Strangely not all tree-dwelling animals have binocular vision like this. Squirrels are an example of an animal that doesn’t.
That’s it for our guide to binocular sight. If you have any questions then get in contact and we’ll be happy to help. There’s lots more coming at SteathyNinjas.com over the coming weeks and months, so stay tuned!
If you enjoyed this post then you’d probably find our post explaining how binoculars function really interesting too.
Our latest guide to choosing a spotting scope is proving really popular. You can read it here.
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