FAMILIAR SIZE 
This room from the collection 
of the Ontario Science Centre 
was designed by Adelbert Ames 
to have distorted proportions.  
Viewing the scene from a 
specific location, however, the 
room creates an illusion that 
we recognize from cues of 
familiar size.

If we are familiar with an 
object’s typical size, our 
memory of its visual angle at 
varying distances could allow 
us to estimate its distance 
because visual angle and 
distance are directly related. A 
classic experiment by William 
Ittelson at Princeton 
University demonstrated the 
possible efficacy of size cues in 
depth perception. Subjects were 
seated in a dark room and were 
asked to look at playing cards 
with one eye. When a card 
twice the size of a normal 
playing card was presented, 
observers tended to say it was 
half its actual distance away. 
When they viewed a card one-
half the size of a normal 
playing card, they tended to 
judge its distance to be twice as 
great as its distance actually 
was. In a variation of this 
experiment, subjects are shown 
a more ambiguous object, such 
as a white sphere. If they are 
told it is a Ping-Pong ball, they 
judge it to be nearer than if 
they are told it is a tennis ball. 
These effects depend upon the 
observer’s stored knowledge of 
the visual angle subtended by 
various objective sizes at 
different distances—–for 
example, that a 3-inch object 
subtends a particular visual 
angle at a particular distance.