We judge depth in many 
ways, some of which are so 
obvious that they hardly 
require mention (but I will 
anyhow). When the size of 
something is roughly known, as 
is so for a person, tree, or cat, 
we can judge its distance--at 
the risk of being fooled by 
dwarves, bonsai, or lions. If one 
object is partly in front of 
another and blocks its view, we 
judge the front object as closer. 
The images of parallel lines like 
railroad tracks as they go off 
into the distance draw closer 
together: this is an example of 
perspective, a powerful 
indicator of depth. A bump on a 
wall that juts out is brighter on 
top if the light source comes 
from above (as light sources 
generally do), and a pit in a 
surface lit from above is darker 
in its upper part: if the light is 
made to come from below, 
bumps look like pits and pits 
like bumps. A major clue to 
depth is parallax, the relative 
motions of near and far objects 
that is produced when we move 
our heads from side to side or up 
and down. Rotating a solid 
object even through a small 
angle can make its shape 
immediately apparent. If we use 
our lens to focus on a near 
object, a far one will be out of 
focus, and by varying the shape 
of the lens--by changing 
accommodation (described in 
Chapters 2 and 6)--we should be 
able to determine how far an 
object is. Changing the relative 
directions of the eyes, adjusting 
the toeing in or toeing out, will 
bring the two images of an 
object together over a narrow 
range of convergence or 
divergence. Thus in principle 
the adjustment of either lens or 
eye position could tell us an 
object's distance, and many 
range finders are based on 
these principles. Except for the 
convergence and divergence, 
all these depth cues need 
involve only one eye. 
Stereopsis, perhaps the most 
important mechanism for 
assessing depth, depends on the 
use of the two eyes together. In 
any scene with depth, our two 
eyes receive slightly different 
images. You can satisfy yourself 
of this simply by looking 
straight ahead and moving your 
head quickly about 4 inches to 
the right or left or by quickly 
alternating eyes by opening one 
and closing the other. If you are 
facing a flat object, you won't 
see much difference, but if the 
scene contains objects at 
different distances, you will 
see marked changes. In 
stereopsis, the brain compares 
the images of a scene on the two 
retinas and estimates relative 
depths with great accuracy.