What Are Monocular Depth Cues? Our Guide!

How does monocular depth cues relate to binoculars and what are they?

Monocular cues are the different cues that each eye uses to determine depth perception. However, these cues are tricked into thinking that an object closer than it is when you use binoculars.

Looking through a pair of binoculars can be a strange experience when you think about the science behind what you are seeing and how you are seeing it.

In a way, the lenses used in binoculars and made to replicate the human eye with a magnified view.

It is this process of magnification that alters our natural depth perception of the world around us as it confuses what we would usually interpret as depth cues.

What is depth perception?

Most people naturally have a depth perception which allows them to see and estimate the distance between two objects.

For example, if a person was to stand at one end of a bridge, then they would be able to see how far away the other side of the bridge was and have a rough estimate of how far that distance is.

This is the use of depth perception.

The perception of depth is able to be determined because of the different functions of each of your eyes. Your eyes do not see the same object exactly the same.

Think of your two eyes like a pair of binoculars. While they may appear to look the same, there is a small difference. With binoculars, it is the addition of a diopter adjustment.

Your eyes will see an image from two different angles and with two different kinds of focus but it is the merging of those images in the brain that allows us to see the world in 3 dimensions.

It is this ability to perceive the world in 3-D that allows us to have depth perception. The skill of being able to see that 3rd dimension means that we can judge how far away an object is.

Monocular cues are the different cues that each eye uses to determine depth perception, which is why they are called monocular cues as it is the cues of one eye.

When you use binocular, the monocular cues clash with the binocular cues aka the cues of both of your eyes rather than just one. This is what allows your brain to process the magnification of an object from a distance.

What are the different monocular cues

There is a reason that they are called monocular cues as in plural rather than just a singular cue. It takes a whole range of different cues for you to be able to have a perception of depth.

Here are the most common monocular cues that you may have heard of through your high school science class:

Motion perception

Have you ever noticed how when an object is closer it seems to move father than when something is far away?

Like when the sun is setting in the distance it seems to take forever to move but if a bug flies in front of your face it feels too quick to catch.

By perceiving how fast an object is moving, your brain is creating a cue for how far away it is.

When using a pair of binoculars to stargaze, you will be able to see the stars and the planets slowly moving as the Earth rotates.

This is because you are tricking your eye into thinking that the stars are closer and so they must be moving quicker.

By removing the binoculars from the equation, you may still be able to see the sky moving if it is a particularly clear night but it will certainly be slower due to how far away it is.

The object's size

Of course, to us, the stars in the night sky appear to be nothing more than a few pinpricks of light on a black canvas but that is not the case.

We know that stars are gigantic balls of gas or other such things so why do they appear so far away to us when the moon is so big?

It is because the size of an object acts as a monocular cue for how far away that object is.

Typically, the bigger the object, the closer we think it is.

For instance, if you have two footballs of the same size and place one directly in front of you with one 4 feet away, the one further away will seem smaller.

The size of the football further away in comparison to larger looking one that is close works as a cue and allows your brain to processes that it is further away as it is smaller.

To test this monocular cue: draw a circle on a piece of paper then draw a significantly smaller circle.

While you may just assume that the smaller circle is just smaller anyway, if you look at it and really process the image, your brain will start to perceive the smaller circle as being further away than the bigger one.

This is the size of an object triggering your depth perception by beings a monocular cue.

Aerial and linear perspective

Just like the size of an object being a monocular cue for how far away that object is, aerial perspective working in the same kind of way.

When an object is further away, it will appear blurry or foggy, especially so if you look at the horizon at any time of day.

This is due to the atmosphere and the dust in the air making it harder to focus on objects further away but still, the blurrier an object is the further away out brain perceives it to be.

The same kind of theory can be applied to parallel lines.

When you look at a painting of a road with a car during off into the distance, the way that the artist has been able to achieve this depth in their work is by gradually bringing the two sides fo the road closer together.

Eventually, they appear to meet at the horizon which we assume to be closer or further away depending on how long it takes for the two sides of the road to meet.

It is this linear perspective that acts as a monocular cue as the closer the lines are the further away they seem to be.

When you are using binoculars, objects in the distance becomes clearer, or at they should be clearer if they have been properly cared for.

Likewise, you will be able to use your binoculars to see that the parallel lines are not touching even though they are further away and so your depth perception is altered.

Lighting and texture

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How the shadows appear on an object can also be a big monocular cue.

Paired with ariel perception, the darker an object appears to be, the blurrier it will appear as there is less colour to determine the clarity of the object. It will therefore appear more distant.

Likewise, as the texture of an object or a view becomes blurrier and unrecognisable, it will appear to be further away from you.

This can be seen when looking into a forest or overlooking a garden. When the trees and leaves are closer to you, you will be able to clearly see their texture and colour.

But as you look out into the forest, the trees will start to blur together and the leaves will become dabs of colour. This acts as a cue for telling you that those objects are further away.

How are binocular cues different to monocular cues

Unlike monocular cues, binocular cues rely on the use of both of your eyes at the same time to understand a view or information about an object in relation to you.

Due to the use of both eyes at the same time, you are able to see far more than you would when just relying on monocular cues.

This means that if you were to go on a safari, rather than having to watch a blurry lion in the distance, you are able to see them in much more detail while still maintaining a safe distance.

It is the benefits of binocular cues and the way that they compensate for the natural monocular cues that humans experience that people love binoculars so much.

They allow you to experience a view in a way that you would not be able to if you were just looking at it with the naked eye.