The visual span or attention window is the area that the brain has selected to analyze during a fixation. During fixation the eyes are relatively stationary, this is when virtually all the visual inputs happen.
For the average reader visual span corresponds to one word; while for the good reader it is about five words at a time. The eyes have the capacity to perceive with sufficient detail text in an area of roughly 30 characters wide by 10 lines high. However as reading is not only seeing, this large span cannot be exploited.