(Top) Retinal image formed by  
an object in a frontal plane: the  
relative proportions between  
points within the object are  
maintained within the image.  
(Bottom) Objects at different  
sizes may subtend the same  
visual angle if they are located  
at different distances. 
 
    If an object is seen in the  
frontal plane, the relative  
proportions between points on  
it are maintained within the  
image, as shown in the top  
figure to the left. The visual  
angle, and consequently the  
size of the image, will vary  
systematically not only with  
the size of the object viewed but  
also with its distance from the  
viewer. For example, a book  
held at different distances in  
your line of sight will cast  
retinal images of different  
sizes, the size of the image (or  
the visual angle that produces  
it) being inversely proportional  
to the book’s distance from the  
eye. This fact is sometimes  
referred to as Euclid’s law of the  
visual angle.