Perceived size varies with viewing distance

Follow these instructions to see how an image of constant angular size on the back of the eye can produce a "perceived" image that varies with viewing distance.

Read these instructions before performing the exercise.

• position yourself at arm's length from the computer monitor
• for 30 sec stare at the black fixation mark located in the middle of the three colored figures in the test plate below, being careful to keep your eye as still as possible; don't look directly at the colored figures
• after 30 sec quickly transfer your gaze to the lower black fixation mark. Quickly judge how big the colored afterimages appear relative to the fixation mark (you can make the afterimages more vivid by blinking your eyes). Now move much closer to the video monitor and see what happens to the size of the afterimages.

The afterimages are temporarily "burned" into your retina by adaptation - once there, they cannot change size. Afterimages appear to be located on the surface of whatever object you're looking at, and their size is determined by the distance from you to that object. If you vary the viewing distance, you'll vary the perceived size of the afterimages even though their actual angular subtense on the back of your eye remains constant. This is called "Emmert's Law."