Another aspect of focus is fixation on a specific physical space with lowered blink rate. As soon as you move your eyes away or blink it lowers your concentration. The text outside of this cone of sight is less sharp. It's like a hunter that finds its prey in a landscape. Seems obvious when you think about it but we rarely consider this when designing our office and workflow.
Edit: love all the replies to this, lots to consider that could really help.