An Exercise for Exploring Diminishing Size

Among the many things that we tend to “misjudge” terribly amid an observational representationalist endeavor is the rate at which apparent size diminishes as viewing distance increases. It’s rather surprising. In looking to very popular perceptual demonstrations like the Ponzo illusion, (C and D), we can see how our perception of size is greatly influenced by context. Conversely, the represented size of an object can have an impact on how the context is perceived.

Take a look at the front and back (A and B) of the cylinder drawing in the image. While subtle, the back (B) appears to be slightly larger than the front (A), when in actuality—just like with the Ponzo illusion examples—the distant elements are the same.

One fun way to explore this in the classroom is to use a piece of Plexiglas (or other rigid transparent material), a marker, and something to measure relatively small distances with–like a ruler or dividers. We gather these items along with a subject that can be placed in a manner so that it may be observed receding into space (here we used a cylinder). We place the Plexiglas directly in front of the object, perpendicular to the viewer’s line of sight. We then have the viewer place themselves in a stationary position to observe the subject through the Plexiglas, have him or her close one eye, and mark off (on the Plexiglas) points that fall along corresponding contours both nearest and furthest from the viewer. When the marks are made, we measure the distances to examine the disparities. Most participants are surprised to see that the differences tend to be more significant than expected.

You can also trace off the entire object to see just how the shape differs from how we think it “should” be. In addition, if your viewing angle is close enough to the horizon, you can observe how objects appear to grow closer to the horizon line as distance from the viewer increases–Another very important depth cue in the communication of space!

Definitely give it a try!

For some further reading related to this topic: