The entire system of linear perspective is based upon the concept of the fixation point. Although we will introduce variations later in this chapter it will be best, for now, to think of the fixation point as being that point that we see with one eye as we stare directly ahead at eye level. Because of the nature of our eye, at any given moment, we can only clearly focus on an area about the size of a quarter. In normal viewing we tend to scan the visual field, moving the sharply focused area from place to place in the visual field, but in the synthetic system of linear perspective we must keep it locked on a single point in space (fixation point). It is an unnatural way to look at things but it is the essential concept underlying linear perspective. Staring this way at the fixation point establishes the line of sight, and the line of sight establishes the orientation of the picture plane because the picture plane is always perpendicular (90 degrees) to the line of sight. The fixation point also establishes the placement of the horizon line. This is a line that is understood to run horizontally through the picture plane precisely at the level of the fixation point.
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