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Do you know how we are able to tell the depth or how near or far something is? Our eyes use clues for depth perception, and there are two kinds of cues—monocular and binocular. The former is the one that we see with just one eye and the latter with both our eyes. Both types of cues essentially help us judge the depth and see things in three dimensions. So, in a more in-depth manner, what is the difference between monocular and binocular cues?

Difference between Monocular and Binocular Cues: A Detailed Look

To understand the difference between monocular and binocular cues, you just have to know what these cues actually are. There are more monocular cues than binocular cues. This is why even when you shut one eye, you are able to tell the distance of things.

Monocular Cues

Below are the different monocular cues.

  • Relative Size
    Each of our eyes uses relative size as a way to perceive the depth of an object in the view. If two things are about the same size, but one appears larger than the other, the brain perceives that the larger object is nearer. It can be applied to two-dimensional as well as three-dimensional scenarios.


  • Absolute Size
    The actual size of an object is also important in determining the depth of it. Small objects in comparison with larger objects do tend to appear farther even if the larger object was placed in the same position.
    The absolute size is directly related to the familiar size: the size we are familiar with. If we already know the size of an object, it gets easier to know if it is near or far. This is one of the cues we subconsciously use when we are driving. Since you know the actual size of the car, you are able to conceive how far the car actually is.


  • Texture Gradient
    One of the most important monocular cues is texture gradient. The clarity of the texture is a cue for depth for our eyes. For instance, if you look at a towel from a distance, you may not be able to see its texture clearly and can perceive that it is at a far distance. Similarly, if you look at a field from a distance, you cannot distinguish one plant from another. As you move closer, you can know the difference.
    In short, the texture gradient becomes clearer as you move towards the scene. Your eyes use the texture to know whether the objects are nearer or farther.


  • Motion Parallax
    The movement perception is another vital aid for depth determination. When you are moving, the objects that zoom by faster are considered closer than those that do so slowly. The perfect example of this cue is when you are moving in a car. The poles on the side of the road move by faster and, of course, are closer. However, trees on the side of the road move slower in comparison as they are farther from you.


  • Linear Perspective
    Parallel lines that meet indicate a farther distance. For example, when you look at the road from a distance, the side of the road appears to be meeting although they are not. Similarly, if you look at very tall trees from the ground, their trunks seem to be meeting on top even though they are separate. This is one way our eyes use previous knowledge with linear perspective to measure distance or depth.


  • Aerial Perspective
    The aerial perspective deals with the clarity of the object itself. The things that are farther on the horizon seem blurry. This is because of the atmosphere—as fog, dust, or moisture obscure objects that are farther. However, the nearer objects are much more distinct and clearer.


  • Overlap
    If the objects in sight are overlapping, the object that is hidden is considered at a greater distance. Your brain considers the occluded object to be behind the less or non-occluded object. This way your eye is able to fathom the distance between these two overlapping objects.


  • Shades
    The way light falls on a subject can also be a clue about their distance. If the light is hazy and dark, that means the object is at a distance from the light, whereas if the light falls brilliantly then the object is nearer.

Binocular Cues

There are two binocular cues, and they are:

  • Retinal Disparity
    Your eyes see a different image, so there is a disparity of image on each retina. Our eyes are at a distance of 6.3cm which allows them to see images from a slightly different angle. This change of angle is a way for our brain to comprehend depth. The simple experiment to see this disparity is to hold a figure right in front of your face. Close one eye followed by the other, and as you do, you will feel as if the finger is shifting, although it is quite stationary.
    How is depth perceived from these changes? Your brain actually receives different angles, sizes, shapes, and positioning of the objects from the two eyes. That information is helpful in determining the distance.


  • Convergence
    When an object is near you, your eyes converge towards each other. They get crossed a bit, and that is because the muscles push them and give them a bit more force. That sends a signal to the brain about the closeness of the object. This means that your eye muscles work harder to focus on something that is nearer as compared with something that is farther.


These are the cues that our eyes use for determining how close or far an object is. We do not use one cue at a time; we use many that work together to send a picture to our brain which then determines the depth. Binocular cues help make the picture more three dimensional. Since more cues just need one eye, you can still perceive the depth of different objects in the view. This is why monocular telescopes are able to see objects at a distance so clearly as many cues are aiding our single eye to perceive the distance.

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