Size Constancy

When some things change they appear to remain the same

Introduction

Your perception of the size of an object depends not only on how big an image it makes on your retina but also on your perception of the distance to the object.

Material

• A partner
• A meter stick

To Do and Notice

Have your partner take 4 paces away from you.

Notice how big they appear.

Hold the meter stick at arms length and use it to measure the height of your partner.

Your partner may have a height of 20 cm measured on the meter stick.

Have your partner take 4 more steps away from you.

Notice how big they appear.
Are they smaller?
Are they half as tall?

Now measure their height on the meter stick.

If they were 20 cm tall, then meter stick shows that they are 10 cm high, one-half as tall.

By holding the meter stick at arms length and measuring the height of an object you are measuring the angular size of the object. The angular size of an object tells you the size of the object on your retina.

See the Angular Size Math Root

What's Going On?

Your perception of the size of an object depends not only on its size on your retina but also on your perception of the distance to the object. By adjusting your perception to allow for distance you can make a better estimate of how big the object "actually" is.

This means that a distant object is adjusted by your eye and brain to look bigger than it actually is.

In order to make the same size image on your retina, a more distant object must be larger.

So What?

When you look down at people on a sidewalk 300 feet or 20 stories below you you say "they look like ants."

Yet when you look at a football player 300 feet away across a football stadium, you don't say, "he looks like an ant."

You have very little experience judging distances when looking down from a building, you have more experience judging distance across the ground.

So you make an adjustment for size constancy when looking at the football player, you do not make such an adjustment when looking at the pedestrians below.

Etc.

Size Constancy provides one possible explanation of the Moon Illusion.