Binocular vision

Binocular vision is the way humans and animals see the world using two eyes instead of one. This special way of seeing helps us understand where objects are and how far away they are. With two eyes, our brain can compare what each eye sees to create a sense of depth, making it easier to judge distances.

There are two main parts to binocular vision: directional vision and depth perception, also called stereopsis. Our eyes work together, either helping or sometimes interfering with each other, in what scientists call binocular interaction.

In medicine, binocular vision is studied through tests and exercises meant to improve how well our eyes work together. In biology, scientists look at how the position of eyes on an animal’s face affects its ability to see depth and direction. In everyday life, binocular vision is important for enjoying stereoscopic images and using tools that help our eyes work better together.

Directional vision

In the science of vision, directional vision explains how our brain understands where objects are in space. When light hits a point on our retina, it not only creates an image but also gives our brain a sense of direction. This helps us see the world from our own viewpoint, combining what both eyes see into one clear picture.

Depth vision

Main article: Stereopsis

Depth perception is how our brain uses the small differences in what each eye sees to understand the world around us. This helps us recognize shapes, see objects hidden by other things, and know how far away things are. The main article on stereopsis explains more about how depth perception works, the space it covers, and how we control what we see by moving our eyes and focusing our attention.

Binocular interaction

Binocular interaction happens when the two eyes work together, making vision different than when using just one eye. This interaction can make vision better or worse. When it makes vision better, it is called binocular summation. In this case, the signals from both eyes help each other, improving how well we can see details, contrast, and brightness. This effect is strongest when both eyes have the same sensitivity.

When vision becomes worse with both eyes open, it is called binocular inhibition. This can happen if one eye is weaker than the other, such as with strabismus or a lazy eye, because the weaker eye can interfere with the stronger one. Eye dominance, where one eye's image overpowers the other, is also a type of binocular inhibition.

Main article: Binocular summation

Perception systems

Our eyes work together in two important ways to help us see the world. One system helps us notice colors and small details, letting us recognize shapes and objects when things aren’t moving much. This system gives us a clear, merged view of what we see and helps us tell how far away things are.

The other system is good at noticing how things are arranged in space, especially when things are moving fast. This system can give us two separate views of what we see, which helps us understand where objects are located over large areas, even if the details aren’t as clear.

Disorders and tests

About 80% of people can see depth, but not all can do so equally well. Several tests can determine how well someone sees depth, and there are exercises to improve depth perception. If one eye does not function properly or is blind, this can cause stereoblindness, a complete lack of depth perception. There are other eye disorders that can affect binocular vision. For example, sometimes the eye muscles do not work properly, causing the images from both eyes to be misaligned. Another example is where one eye is dominant, so that the signals from the other eye do not come through in the binocular image.

Binocular vision abnormalities are among the most common visual disorders. They are usually accompanied by symptoms such as headache, eye strain, eye pain, and blurred vision. Approximately 20% of patients who come to an optometry clinic have binocular vision defects. Lazy eye or amblyopia is a condition where the vision in one eye is weaker than the other. Squint or strabismus is when the eyes do not look in the same direction, which can affect how well a person sees depth. Aniseikonia is when the images in each eye are different sizes.

Lazy eye

Main article: Amblyopia

Squint

Main article: Squint

Aniseikonia

Main article: Aniseikonia

Stereopsis tests

In stereopsis testing, images are used to measure how well someone can see depth. There are two common types of tests: random-dot stereotesting and contour stereotesting. Random-dot stereotesting uses images with hidden shapes, while contour stereotesting uses images with clear edges.

Random-dot stereo tests

Main article: Random dot stereogram § Random dot stereo tests

Contour stereotests

Vision therapy

Main article: Vision therapy

Other disorders

In animals

Vision helps animals understand their surroundings, recognize objects, and move around. Some animals, like fast birds and pilots, use vision from one eye to guide quick movements. Humans and some animals also use both eyes together to see depth, which helps them judge distances. This is especially useful for animals that wait for prey before striking.

Having eyes on the front of the head, like in primates and birds of prey, often helps with seeing depth. Animals like rabbits have eyes on the sides to see more of their surroundings. Eye movements also play a role — some animals move both eyes together, while others move them separately. For example, a chameleon can look in different directions with each eye but can also focus both eyes on one object when hunting. Many different animals, including birds, mammals, and even some sea creatures, have this depth perception ability. The distance between an animal’s eyes affects how well it can see depth.

Applications

Applications for binocular vision are aids for binocular vision, aimed at making, recording, and viewing stereo images.

The binocular microscope and binoculars can magnify images. By increasing the distance between the front lenses of the binoculars and decreasing the distance of the front lenses of the microscope, the perceived depth is in proportion to the magnification. In the course of history, various types of stereoscopes have been developed with which specially prepared stereo recordings (stereograms) can be viewed in 3D, both at home and in the cinema. The most recent development is the VR glasses.

Binocular vision has many practical uses. Tools like binocular telescopes and binocular microscopes can make distant objects or tiny details appear larger and closer. Even without perfect depth perception, using both eyes together can make images appear clearer and brighter. This happens because the brain combines information from both eyes, making the scene seem more vivid.

Stereo images capture what each eye sees separately and then show them together to create a 3D effect. These images can be recorded at the same time or one after the other using special cameras. When viewed correctly, they give us a strong sense of depth, much like how we see the world in real life. Stereograms are used in research, entertainment, and education to help us understand and experience depth perception in new ways.

History

Alhazen, an Arab scholar from the 11th century, first suggested that we see because light reflects off objects and enters our eyes, creating perceptions in the brain. In the 19th century, Ewald Hering expanded on this idea, proposing that each eye sees direction and introducing the concept of a "cyclopean eye"—as if we view the world from a single point between both eyes. This helps explain how we can see things clearly, whether they appear as one image or two.

Hering also studied how our two eyes work together, showing that images from both eyes can merge into one. He conducted experiments to understand how we perceive depth and direction. Researchers later discovered that special cells in the brain help us combine what each eye sees, allowing us to understand where objects are in space. This work laid the foundation for understanding how our vision works with two eyes.