Over millions of years of human evolution we developed various ways to perceive depth & distance as it was necessary to survive in the wild and to navigate through the jungles.

We may now live in fancy & safe neighborhoods and work in high-rise buildings but our ability to perceive world around us played an important role when our ancestors lived in the jungles. Ability to perceive depth, distance, proximity & orientation was crucial for survival. Since our retina gets a two dimensional view of reality around us, our brain uses certain shortcuts to predict these crucial parameters. An understanding of how we perceive depth can be very helpful in creating compelling images. Artists mastered these principles centuries ago and made some mind-blowing realistic work. They used color shading, distance fog, perspective and relative size to make their work look more realistic. Same knowledge can be put to some good use by a smart photographer. Before we go further, I would like to mention that the best way to explore this article is to read about a principle and then try to imagine how you can incorporate it while composing a photograph, maybe read more on the internet or make a photograph using the principle. Ready? OK, here we go.

There are two categories of Depth cues, namely, monocular & binocular. Monocular cues work even if vision is only one eyed while binocular cues make use of the two eyes. This article will focus more on Monocular cues since our DSLR camera systems use a single lens to record an image and therefore is more relevant to our preferred format of photography.

1. Perspective 

Two parallel lines seem to converge at a distance. Therefore, if in a photograph there are converging lines it gives a better sense of depth. A very good example is train tracks. This foreknowledge also forms a bias in our visual system. We know that objects near to us look bigger and away from us smaller. So if we place converging lines and place two equal sized objects along the line then the object behind will look bigger, since our brain thinks "oh, if the projection is this big then the actual size must be much bigger since it is kept at a distance but still looks so big. Following image will explain it better: 

 

Both yellow bars are of the same length but due to presence of other cues the bottom bar looks shorter than the one on top.

2. Depth from motion

The projection of an object on retina grows in size as the object moves closer to the viewer. In other words, changing size is perceived as motion. This is used in animation frequently. A person or object moving towards the viewer gets bigger while it gets smaller as it recedes away.

(Photography by: Ashish Bharti)

3. Kinetic Depth Effect

This is about perceiving depth information from the shadow of an object in motion (rotation). If a solid 3-D object is casting shadow on a flat surface and if the object starts moving (rotating or revolving) then the visual system gets sufficient data to guess the true shape of the 3-D object only from the shadow. This cue helps to understand shape of an object.

4. Motion Parallax 

If viewer is moving then objects closer to the viewer seem to move past quickly as compared to far off objects seem to move much slower. This can be observed while driving a car or travelling on a train. Following Image illustrates this:

(Photography by: Ashish Bharti)

5. Size 

Two objects which are known to be the same size but absolute size is unknown, for example, two car Tyre, one appears larger than the other then that means to our brain that larger one is closer while smaller one is farther away. Also, if an objects size is known from previous experience then depending on how big or small it appears depth can be perceived. Even if the absolute size is unknown and looks smaller then it will be perceived farther away then a bigger object presented at the same location. Following image shows this bias of our perception: 

Both the balls are physically of the same size however due to presence of other cues (converging lines, pattern) our brain thinks that the smaller ball is in front while other ball which is making an equal size image on retina must be much larger since it looks equally big in spite of being kept behind.

Size is a very important concept to master, if learned well, size can be used to depict isolation & loneliness or group & belonging. 

6. Distance Fog

Atmosphere has a property to disperse light and therefore objects near to the viewer have high contrast and color accuracy while objects farther away appear to have low contrast and colors seem to shift towards blue. Objects differing only in their contrast with a background appear to be at different depths. Cooler color tones (bluish colors) appear to recede away while warmer tones (red, yellow, etc.) appear to be close to the viewer. Following image illustrates this effect:

(Photography by: Ashish Bharti)

7. Order

If one object is closer to the viewer and another kept behind the first object then the object which is closer will, partially or completely, hide the second object. IN other words objects in front will hide the objects kept behind. Following image explains this:

It can be clearly seen that which ring is in front and which ring is behind. The ring in front hides the ring behind.

8. Texture or Gradient

Pattern / texture is visible at close distance however becomes indistinguishable at farther distance. Same is true with lighting, light falls off exponentially as the distance from the source increases. Below is an example:

(Photography by: Ashish Bharti)

9. Shadow

Shadow and light reflection serves as a cue to determine the shape and position of an object. There is an interesting catch to this! This applies to all the objects but not to human faces. Look at the following two images:

 

The image on the left shows effect of light and shading. If you assume that light is falling from top of the image then top two circles and bottom 3 looks extruded out of the plane of the image and others look like a bowl receding into the plane of the image. Interestingly, if you assume that light source is located at the bottom of the image, your perception of concave and convex will switch! However, the same is not true in case of human faces. The image on the right shows that indifferent to lighting & shading our brain knows that our faces are always convex and that's how we see. To see a small video click Hollow face Illusion.

10. Horizon

Objects at the horizon appear to be far away while if placed above or below horizon the same object looks closer to the viewer.

11. Shadow Stereopsis

Shadow from an object is also analyzed in visual data from both the eyes and used by our brain to estimate the depth. 

All the above cues which help human beings to perceive depth can be used in photography to fool our brain but there are some more cues that our brain uses which cannot be used in Photography, however, knowledge of these can still be helpful in understanding how important depth perception is for humans. 

a. Accommodation

When our eyes shift focus from a near object to on an object that is far away there are eye muscles, known as ciliary muscles, which contract to change the focal length of the eye and this muscle contraction information is utilized by our visual cortex system to estimate depth. Visual cortex is part of the brain, located at the back of the head, responsible for vision in humans. 

b. Binocular parallax (Stereopsis)

Images formed by each of the two eyes are compared and the difference between the two images is analyzed by human brain. This difference will be small if the object is at a large distance while the difference will be great if object is close to the viewer. 

This technique is used to make graphics which produces a visual effect of seeing a different hidden image if a viewer focuses on the primary image. Below is a sample image:

c. Convergence

When two eyes focus on an object they both converge and a muscle known as extraocular muscles ensures the movement of the eye balls, this information is sent to the visual cortex system to estimate depth.

 

Snapoholic by Snapoholic.com