Line of Vision
The line of vision is an imaginary line that extends in a straight path from the eyes of an observer to an object. While eye level comprises everything within the observer’s field of vision at the height of that person’s eyes, the line of vision connects the observer’s eye with a particular object. For a drawing’s perspective to remain consistent, you must maintain a single line of vision.
The picture plane is an imaginary vertical plane fixed between observer and object at a right angle to the observer’s line of vision. This line of vision runs through the picture plane, and the angle of the line of vision determines the position of the picture plane.
Think of the picture plane as a sheet of glass through which you see your subject. If you could trace the image on the “glass” picture frame, the image would look a lot like the image you wish to create on paper.
To maintain a consistent perspective when drawing, your eye level, line of vision, and picture frame must remain constant. All of these elements are interconnected; a change in any one of them will significantly alter the outcome of your work. So what do you do if the subject you wish to draw in perspective is too large for you to see without moving around?
When the subject of your drawing is too large to see without moving your head or shifting your eyes, then you might draw only a segment of that subject. However, if you wish to include an entire large subject in your drawing, you must position yourself as best you can to view the subject without moving your head or eyes.