I was reading about perspective drawing in Bridgman (Complete Guide to Drawing From Life), and was puzzled about a diagram like this. Remember, the head is kind of a cube (8 inches high, 6 inches wide and 7.5 inches deep).
Of course, what we see in the drawing above is two-point perspective. However, I could not understand why Bridgman used a circle around the block of the human head. When I tried to replicate similar drawings on the computer, I had to rotate the head, instead of translating it. Then it dawned upon me. The red circle is the field of vision. The human eye can only see sharp in a narrow viewing angle. Anything outside the primary field of vision is peripheral vision, and not considered important for observation. This means to see the world, we change our field of vision (the red circle), and build a complete picture by combining observations into a whole picture.
This means objects don’t translate left or right, but rather you turn your field of vision, as an action. If you think about this, it makes sense, but I just couldn’t wrap my head around it until I did the computer experiments. So, because the field of vision is so narrow in humans, when we say we see something on the left or on the right, doesn’t mean that something is moved to the left or to the right, but rotated left or rotated right.
So if we look at someone, it makes not difference for the perspective if this person is turning his head or moves to the right or left. If this person moves left or right, we need to turn our heads to see the person in the new location. We must move the red circle, our field of vision, in order to see this person in a different location. Of course, in conversation (and how we think about the world) we use location in terms of position, how to reach an object, not how to look at it (because we already have seen the object, so that wouldn’t make much sense).
I’m glad I sort of grasped that concept of perspective drawing.