Computer eye strain and computer vision syndrome are caused by our eyes and brain reacting differently to characters on a computer screen than they do to printed characters. Our eyes have little problem focusing on printed material that has dense black characters with well-defined edges. But characters on a computer screen don't have the same degree of contrast and definition.
Words and images on a computer screen are created by combinations of tiny points of light (pixels), which are brightest at the center and diminish in intensity toward their edges. This makes it more difficult for our eyes to maintain focus on them. Instead, our eyes want to drift to a reduced level of focusing called the "resting point of accommodation" or RPA.
Our eyes involuntarily move to the RPA and then strain to regain focus on the screen. This continuous flexing of the eyes' focusing muscles creates the fatigue and eye strain that commonly occur during and after computer use.