How Two Eyes Create 3D Vision and Depth Perception
It is quite baffling that we have two eyes and require the use of two eyes simultaneously, even though we still have a sense of vision if we cover one of our eyes. The reason why we have two eyes is to enable two things in our brain, namely depth perception and an increased field of view. Typically, a pair of functioning human eyes are known to have a field of view of about 170 degrees, and humans are very capable of accurately determining the relative distance between the objects kept in front of them.
The Science of Depth Perception
Even though we have two eyes, we only see one image at a time. Our brain only sees images sent through one eye instead of perceiving and making sense of the information posted by both the eyes at the same time. So, what happens in our brains is that it alternates between the data sent from two eyes to allow us to see the third depth dimension. The ability to see the distance between the two objects in the field of vision is known as depth perception.
Since both the eyes are very close to each other, both of the eyes see almost the same picture. It is the job of our brains to make sense of the two images sent into our minds.
So, the process of sight begins when first, the light from the object strikes the lens of our eyes. This forms an inverted image of the object on our retinas. In the next step, the picture which is created with the help of the optic nerve, is sent to the brain for processing.
We, humans, have a significant advantage due to our binocular vision. Our binocular vision allows us to determine the exact position of any object placed in front of us.
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Understanding our Eyesight Through Some Activities
There are certain activities that you can conduct to check why we require two eyes instead of one eye.
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To verify that we see only from one eye at a time, let’s try out this activity. Close one of your eyes and notice the brightness level of the images you see. Now, keep switching between the eyes. Do you feel a noticeable change in the brightness or the field of vision of the image you see? If your eyes are working correctly, you shouldn’t be able to ascertain any differences in the pictures you see from the two different eyes. This justifies the claim that we humans only see from one eye at a time, and the speed at which the brain switches between the two eyes is very fast for us to notice.
To understand the depth perception, there is a straightforward activity you can set up. You will need various objects, each with different shapes and sizes. Now, ask the volunteer to close their eyes while setting these objects in front of them.
Your setup should look somewhat like in the picture shown above. Once you are done with the setup, ask the volunteer to open one of the eyes. Give them a moment to get accustomed to their surroundings. Now, you have to point to any object and ask them to touch it. You will notice that your volunteer might almost always reach the objective that you ask them to, but they will struggle to touch it.
Now repeat the same experiment with both eyes open. The struggle would not be present anymore. Why was the volunteer struggling to reach the objects with one eye? Our brains rely on both of our eyes to determine the exact position of any object it sees. Since one of the eyes was closed, the brain only received half of the information. With half of the data, the determination of the position of any object is complicated, and the depth perception of our brain would be flawed. This is one of the main reasons why humans have two eyes and why we need the image from two eyes to function properly.
Purpose of having Two Eyes
Close your right eye and take notes mentally on your field of vision using only your left. Close your left eye and repeat the process. Now that you have stopped closing your eyes, take a moment to appreciate the entire field of vision you have thanks to your TWO EYES!
One of the most significant advantages of having two eyes is this. So to speak, we have twice as much vision. This binocular vision has a field of roughly 170o for an average pair of well-functioning eyes.
Try a new experiment. Now that both of your eyes are open, take note of how bright your current surroundings are. Alternate between keeping one eye open and the other. Does the brightness level alter (and does the field of view shift as well)? It most likely is not if the eyes are not working properly. So, what is going on here? Your eyes are actually only looking through one eye at a time, but they alternate fast. It happens so quickly that you do not even realize it. This method is also ideal for people who have double vision.
Because of this double eye vision, the problem of parallax error in vision is greatly decreased. It also improves our depth perception, offering us a significant evolutionary benefit. In addition, our ability to recognise faint objects has improved. Humans can enjoy stereoscopic vision thanks to our "two" eyes. Some two-eyed animals, such as rabbits and buffaloes, have a 360o field of vision but not stereoscopic vision. Only creatures having two eyes in the front of their heads, such as humans, have this advantage.
The Human Eye also has a pleasing appearance. Asymmetry is always seen as less appealing than symmetry. This criterion is met by having two eyes. Apart from the enormous advantages listed above, having two eyes has another incredibly essential benefit. There is always a spare if one dies!
What is Diplopia?
Diplopia, or double vision, is a type of visual impairment that occurs when the two eyes are misaligned or when a visual deficit stops the brain from accurately interpreting the information it gets from the two eyes and resolving it into a single image. Double vision can be caused by an injury to one of the small muscles that control eye movement, which can be corrected with simple strengthening exercises, or it might be a sign of nerve damage caused by a more serious ailment.
What is Amblyopia?
Amblyopia, sometimes referred to as a lazy eye, is a condition in which one or both eyes acquire abnormalities. These anomalies begin to appear in infants or children under the age of eight. Amblyopia is more likely to occur in newborns who are underweight or even prematurely born babies. This disorder arises when a portion of the brain responsible for vision processing fails.
FAQs on Why Do We Have Two Eyes?
1. What are the primary advantages of having two eyes instead of one for human vision?
Having two eyes, known as binocular vision, provides several significant advantages over having just one. The key benefits include:
- Wider Field of View: Two eyes allow us to see a broader horizontal area. A single eye offers about a 150-degree field of view, whereas two eyes together increase this to approximately 180 degrees, helping us detect objects in our periphery more effectively.
- Depth Perception (Stereopsis): Each eye captures a slightly different image of the same object. The brain fuses these two images into one, creating a three-dimensional perception. This ability, called stereopsis, is crucial for judging distances and navigating our environment accurately.
- Visual Redundancy: Having a second eye provides a 'spare' in case one is injured or impaired. This ensures that we do not lose our sense of sight completely if one eye is damaged.
2. If we have two eyes, why do we see only one unified image instead of two separate ones?
We see a single image because of a process in our brain called binocular fusion. Although each eye captures its own distinct image from a slightly different angle, the visual cortex in the brain processes and combines these two inputs into a single, coherent, three-dimensional image. This neurological process is so efficient that we are not consciously aware of the two separate images being received.
3. What is the importance of stereopsis, and how does it work?
Stereopsis is the technical term for depth perception achieved through binocular vision. Its importance lies in our ability to perceive the world in three dimensions (3D) and accurately judge the distance, size, and depth of objects. It works because our eyes are separated by a few centimetres, causing each to see the world from a slightly different perspective. The brain analyses the small differences (binocular disparity) between these two images to calculate depth. This is vital for everyday tasks like catching a ball, driving, or walking down stairs safely.
4. How would our perception of the world be different if we only had one eye?
If we only had one eye (monocular vision), our perception of the world would be significantly different. The most notable change would be the loss of true depth perception (stereopsis), making the world appear flatter, much like a photograph. While we could still judge distance using other cues (like object size and perspective), tasks requiring precise distance judgement would become much harder. Additionally, our horizontal field of view would be reduced, making us less aware of our surroundings.
5. Why do predator animals like humans have forward-facing eyes, while prey animals often have eyes on the sides of their head?
The placement of eyes is an evolutionary adaptation related to an animal's role in the food chain. Predators, such as humans, eagles, and cats, have forward-facing eyes. This provides a large area of binocular overlap, resulting in excellent depth perception, which is essential for accurately judging the distance to their prey. In contrast, prey animals, like rabbits and deer, often have eyes on the sides of their heads. This placement gives them a wide, panoramic field of view, allowing them to spot predators approaching from almost any direction, even if it means sacrificing some depth perception.
6. From a physics and biology perspective, why did humans evolve with two eyes and not more?
For mammals like humans, having two eyes is an optimal evolutionary strategy. Two forward-facing eyes provide the perfect balance between a reasonably wide field of view and excellent depth perception, which was a major survival advantage for our ancestors. Adding a third eye (or more) would offer diminishing returns; it would not significantly improve our depth perception but would require substantially more brainpower to process the extra visual information. Therefore, two eyes represent the most efficient and effective arrangement for our biological needs.
