• Background
  • Instructions
  • Illustration


Relative size refers to the fact that the more distant an object, the smaller its image will be on the retina. Therefore, if there are two identical objects, the one that is farther away will be the one that has a smaller image on the retina. For example, if we assume that the two street lamps in the photograph below are the same size, then the street lamps with a smaller image on the retina must be farther away from the viewer. As the image of the closest street lamp is the largest in the picture then we assume that it is the closest to the viewer. Note that the street lamps in the background does not look abnormal in any way. Despite creating a smaller image on the retina, the street lamps do not look oddly small. The normal size of the more distant street lamps is due to a mechanism called size constancy, which will be discussed in another activity.

Photograph illustrating relative size.  There are light poles with wreaths on them.  As the poles appear farther, it is clear that they are smaller in the image.

In this activity, you can manipulate the relative size of circles and see how that alters both the perception of depth and size. You can add and remove other depth cues to see how they might alter how you see these circles..


Full Screen Mode

To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.

Illustration Tab


On the Illustration tab, you can adjust these parameters:

Relative Size: Adjust the size of the two middle circles. Notice both the changes in the apparent depth and the perceived sizes.
You can select from the following pictorial depth cues add them to this scene and see how they might alter the way you perceive the circles. Choose from:

Relative Height: causes the left-most circle to move down and the right-most circle to move up.
Shadow: Add a shadow to the two outer circles to simulate that they are off of the screen.
Linear Perspective: Addes lines that can be parallel for now depth, or approach each other as they appear to recede.
Texture Gradient: Add a gradient that can indicate a receding area.


Pressing this button restores the settings to their default values and allows you to adjust speed and relative size. It also resets the counter before you can indicate if you have the objects arriving at the same time.