Distance and Size Perception

Demonstrations (Direct Links)

Demonstration 6.1 Using Pictorial Depth Cues
Demonstration 6.2 Motion Parallax
Demonstration 6.3 Kinetic Depth Effect
Demonstration 6.4 Stereoscopic Pictures
Demonstration 6.5 Emmert's Law
Demonstration 6.6 Size Constancy
Demonstration 6.7 Influence of Surrounding Texture on Size Constancy

Before You Start


Perceiving a Three-Dimensional World

Link - You'll find helpful information about viewiing a 3D world at (Rachel Cooper).

Monocular Cues to Depth

Pictorial Depth Cues


Linear Perspective

Size Cues

Texture Gradient

Atmospheric Perspective

Shading (Attached and Cast Shadows)

Link - Dan Kersten (University of Minnesota) has provided a page of demos, including a nice demonstration of the impact of shadow on perceived depth in motion.

Height Cues

Pictorial Depth Cues on Two-Dimensional Surfaces

Demonstration 6.1 Using Pictorial Depth Cues

Draw a picture to illustrate pictorial depth cues. You should use as many cues as you can. (Don't worry if you have no artistic abilities!) Next, turn on a television when you can view a cartoon show. Does the motion displayed in the cartoon lead to a richer sense of depth?

Link - Victor Vasarely is an artist who created a sense of a 3D world on a 2D canvas. Vega Nor (1969) is one example.

Moncular Depth Cues Involving Motion

Motion Parallax

Demonstration 6.2 Motion Parallax

For this, you’ll need a car and a driver (since your attention will be directed out the side window and not on the road).  Go for a drive on a nice scenic route.  Anything with some depth.  Roads lined with lots of buildings will not work here.  Once you’ve found a stretch of road, look out the window.  Notice how objects in the distance move in relation to those closer to you.  The difference that you perceive in the movement between near and far objects is called motion parallax.  Motion parallax provides important monocular depth perception information. John Krantz (Hanover College) has developed a simple demonstration of motion parallax.

Kinetic Depth Effect

Demonstration 6.3 Kinetic Depth Effect

Take a pipe cleaner or a paper clip and bend it into a clearly three-dimensional figure. Find a piece of paper and a lamp. Place the figure between the lamp and the paper and notice that the shadow of the figure, as seen on the paper, looks flat and two-dimensional. Now rotate the figure and notice how it suddenly appears to be three-dimensional.


Steven Gilbert (SUNY Oneonta) developed a simple computer program to illustrate a neat kinetic depth effect. You can read about this illusory sense of depth at: Gilbert, S. (1991). A new kinetic depth illusion for introductory psychology and sensation and perception courses. Teaching of Psychology, 18, 55-56. You'll find a similar demonstration on YouTube (note that the direction of rotation will shift as you watch the video).


Binocular Disparity

Binocular Disparity and Stereopsis


Using Binocular Disparity to Create Depth in Pictures

Demonstration 6.4 Stereoscopic Pictures

Print out the simple jpg image below and place a piece of cardboard on the dotted line perpendicular to the paper. Rest your nose on the cardboard (yes, you are gong to look a little ridiculous). Focus each eye on the figure on the appropriate side of the cardboard. Try to fuse these two images into one single image. You may find it helpful to try converging your eyes by looking slightly cross-eyed. When you achieve a single image, it should look three-dimensional. Once you have achieved that, you might want to try the other stereoscopic image. Feel free to take a look here and try other stereoscopic images.



Binocular Rivalry

Eye Muscle Cues to Depth

Approaches to Distance Perception

The Direct Perception Approach

The Empiricist Approach

The Computational Approach

Perceiving Three-Dimensional Objects

Navigating a Three-Dimensional World

Physiological Bases for Depth Perception

Size Perception

Factors Influencing Size Perception

Determining the Size of Objects at the Same Distance from the Observer

Determining the Size of Objects at Varying Distances from the Observer

Demonstration 6.5 Emmert's Law

First, attach a piece of paper to a wall several feet away from where you are sitting.  Now, fixate on the center of the black circle below for about one minute.  Then, look at the white space to the right of the circle.  You should see a bright-looking afterimage. Note the size of the afterimage.  Now, transfer your gaze to the white paper that you just hung on the wall.  Note whether the afterimage seems to have grown. Tip: If the afterimage seems to fade during your experiment, blinking rapidly will restore it.

Black Dot

Link - Dan Kersten (University of Minnesota) has provided a page of demos, including a nice demonstration of the impact of perceived distance on size perception.

Size Constancy

Demonstration 6.6  Size Constancy

Take your pen and hold it about 1 inch in front of your eyes.  Notice the size of the visual angle that it occupies and think about how big the retinal size must be for a pen held this close.  Now move the pen out to about 12 inches.  Notice how the visual angle is much smaller; the retinal size is also much smaller.  Now prop up the pen and walk across the room.  The visual angle is now extremely small, and the retinal size is also extremely small.  Think about how the pen looked to you.  Did it seem to shrink to doll-sized proportions as you walked away from it?  In fact, it seemed to stay a constant size, despite the fact that the retinal size was much, much smaller when the pen was viewed from across the room.

Size ConsistancySize Consistancy 2
Photo by R. Oldmixon

Demonstration 6.7 Influence of Surrounding Texture on Size Constancy

Find a long, flat area with a noticeable texture pattern (such as a tile floor, a sidewalk, or a rug with a regular geometric pattern).  Take 2 same-sized sheets of paper and place them about 1 foot and 10 feet away from you.  Notice that both sheets cover the exact same number of texture units.

Texture and Size Consistency
Photo by R. Oldmixon


Illusions of Distance and Size

Ambiguous Depth or Distance Information


Illusions Involving Line Length or Distance

Link The Exploratorium has a number of online exhibits of interest, but for our purposes, you should look at Changing Illusions.

With the Pulfrich Phenomenon, you'll perceive a swinging object (moving back and forth in the same plane parallel to your eyes) as moving closer and farther away (i.e., moving in depth). To produce the phenomenon, you need to place a dark filter over one eye. You can experience the phenomenon yourself (with the appropriate lens/filter) using Mark Newbold's site. You can see a visual explanation of the phenomenon here. You can also achieve this illusion of motion in depth using the filter over one eye and looking at a television showing "snow" (i.e., no video signal).

Explanations for Line-Length and Distance Illusions

Illusions Involving Area or Size

Link Richard Wiseman and one size illusion

Context is very important for perception. For instance, the picture on the left below depicts an Adirondack Chair. (The chair itself may be found in the Kingsbrae Garden at St. Andrews-by-the-Sea, Canada.) Note that in this context, you probably see the chair as "normal." The actual size of the chair is more readily apparent when it is occupied, as in the picture on the right below.

Photos by M. A. Foley

Test Yourself

1. Even before reading this chapter, you may well have been able to list several distance cues, such as familiar size. Other depth cues may not have been so obvious. Describe each of these cues, and point out their importance in theories about distance perception.

2. Many of the distance cues we discussed are “relative” cues to distance, because they do not seem capable of enabling us to measure distances exactly. Nonetheless, people are relatively good at estimating nearby distances. Try to figure out which cues, if any, might give rise to absolute, rather than relative, distance perception. Then, try to figure out how the depth cues might work to provide us with absolute information about nearby distances.

3. Artists interested in realistic portrayals of depth must solve the same problem facing the visual system—they must create a perception of three-dimensionality on a two-dimensional surface. Trompe l’oeil art is particularly effective in this regard. Is trompe l’oeil art illusory? How many of the illusions discussed in this chapter seem to arise from the same principles used to represent depth pictorially?

4. Describe the binocular cues to depth. Why are random-dot stereograms and autostereograms important? Given all that you now know about the cues to depth, how important are the binocular cues relative to the monocular cues? (Think of the spatial circumstances under which binocular cues are useful.)

5. Summarize the direct perception approach, the empiricist approach, and the computational approach with regard to distance perception. In what ways is the computational approach similar and dissimilar to the other two approaches? Which approach seems best able to explain your experience of depth? Why?

6. How accurate is your representation of space? Close your eyes and try to reach out and grasp nearby objects or point to more distant objects in the room, estimating their egocentric distance in feet. You should find that you are fairly accurate. What factors give rise to your sense of space? Do you agree with Berkeley that your sense of space emerged from experience, or do you think that space perception is innate? What evidence would you use to support your claim?

7. One tactic suggested to people trying to lose weight is to put their food on a small plate. What factor in size perception suggests that a person might think that more food was on the plate? What other factors might be used to bias size perceptions?

8. Why are constancies important, particularly size constancy? To answer this question, imagine trying to move around in our world if constancies did not exist. Try to describe a specific, concrete experience (e.g., catching a ball).

9. What mechanisms seem to be important in maintaining size constancy? How is Emmert’s law useful in understanding size constancy? If distance cues were unavailable but other cues were present, do you think that size constancy could be maintained? Why is the size of the moon not constant?

10. Anastasi found that a parallelogram appears to be larger than a square of equal area. Examine the theories provided to account for visual illusions to see if you can find one that might give rise to the misperceived area of parallelograms. (Hint: Look at the top of the Necker cube in Figure 6.23.) Which explanations of visual illusions seem best able to deal with each of the illusions in this chapter? Pay particular attention to the area illusions.


Teaching Materials

Thomson Higher Education has published two very useful CD-ROMs. John Baro (Polyhedron Learning Media) has developed Insight: A Media Lab in Experimental Psychology [see Measuring Depth Perception and 3D Pictures] and Colin Ryan (James Cook University) has developed Exploring Perception [see Module 4].

Lafayette Instruments The Depth Perception Apparatus lends itself to several demonstrations and even simple experiments that the students themselves might conduct. The Illusionator Set contains several illusions, but the Ames rotating trapezoidal window is always a winner! You might also try constructing a special rotating "window" by taping together two trapezoidal windows at the wide end, as demonstrated by Julian Hochberg at one EPA conference. After viewing the regular trapezoidal window, see if your students can accurately predict the perceived motion of the double window. (Fineman's book shows how to construct the original trapezoidal window, so just double the recipe on your photocopier.) You can find an online version of the Ames Rotating Trapezoidal Window at the Exploratorium exhibit site.


ViewMaster by Fisher-Price. If you don't have access to an old stereoscope, this child's toy (ahem) is an excellent means of illustrating the effectiveness of binocular disparity.

Al Seckel has put forth a number of compilations of visual illusions as well as a web site (with links to his books).

Walter Beagley (Alma College) has developed Eye Lines, which still works on non-Intel Macs and PCs. The package may be used for a variety of purposes, including using classical psychophysical methods to assess the magnitude of illusions. Keep your eyes open for a forthcoming revision to the program.

Beagley, W. K. (1993). Eye Lines: Generating data through image manipulation, issues in interface design, and the teaching of experimental thinking. Behavior Research Methods, Instruments, & Computers, 25, 333-336.

Wurst, S. A. (1994). Generating stereoscopic displays with Eye Lines: Applications in instruction and research. Behavior Research Methods, Instruments, & Computers, 26, 148-150.

Link - If you are interested in demonstrating SIRDS (autostereograms), a good starting point is the Magic Eye site. If you'd like to create your own (as well as anaglyphs), you can use 3D Maker by Sandy Knoll Software. Stuart Inglis has produced a page of FAQs about SIRDS. There's also Vern's SIRDS Gallery for your viewing pleasure.

Link - Michael Bach has created a site of visual illusions that contain some illusions of distance and size.

Link - A number of artists create realistic works that capture 3-D space on a 2-D canvas. However, you'll likely enjoy these atypical artists who manage to create trompe l'oeil art (at least when seen from a particular perspective): Julian Beever, Edgar Müller and Kurt Wenner.

Link - You will find a very extensive set of illusions, including illusions of size and distance, at SandlotScience.

Link - Take a look at Mark Newbold's Stereo 3-D Stuff. You already know that Necker Cubes are wonderful demonstrations, but wait until you (and your students) see Mark's Necker Cube animation.

Link - The Smith-Kettlewell Eye Research Institute maintains a research page that contains relevant links.

Recommended Readings

Jenkin, M. R. M. & Harris, L. R. (Eds.). (2006). Seeing spatial form. New York: Oxford.

Julesz, B. (1995). Dialogues on perception. Cambridge, MA: MIT Press.

Julesz, B. (2006). Foundations of cyclopean perception. Cambridge, MA: MIT Press.

Morgan, M. (2003). The space between our ears: How the brain represents visual space. New York: Oxford.

Patterson, R. & Martin, W. L. (1992). Human stereopsis. Human Factors, 34, 669-692.

Pizlo, Z. (2008). 3D shape: Its unique place in visual perception. Cambridge, MA: MIT Press.

Plumert, J. M. & Spencer, J. P. (Eds.). (2007). The emerging spatial mind. New York: Oxford.