As we look at close objects, our eye muscles move our eyes inward toward the nose. This convergence lets the eyes project the the light from both objects to the same relative place on the retina of each eye. If we didn't have accurate convergence, we would see double images (Tychsen 1992).
Vergence And Distance
Our eyes also have a resting point of vergence. This varies among individuals, but averages around 45 inches when looking straight ahead (Heuer 1989). Strain on the muscles controlling eye movements occurs when we view objects closer than the resting point. The closer the distance, the greater the strain.
Two studies have shown that the resting point of vergence has an even greater impact on eyestrain than the resting point of accommodation.
In the first study, Jaschinski-Kruza (1988) divided subjects into two groups: near and far.
The first (near) group had resting points of accommodation of around 20 inches. The second (far) group's resting points of accommodation averaged 40 inches.
Both groups worked on computers at viewing distances of 20 inches and 40 inches. As expected, the near group had less eyestrain working at 20 inches than the far group. But both the near and far groups had less eyestrain at the 40-inch distance. Both groups judged the 20-inch monitor distance as "too near," and both groups accepted the 40-inch distance.
When Jaschinski-Kruza measured productivity, he found that both groups had greater productivity at the 40-inch distance.
Although their resting points of accommodation were different, both groups had far resting points of vergence. Jaschinski-Kruza concluded that increased convergence seems to be crucial in causing visual strain.
Owens and Wolf-Kelly (1987) found that after one hour of near work, the resting point of both accommodation and vergence shifted inward toward the eyes. The amount of the shifts depended on the positions before the near work: Subjects who began the tests with far resting points had the greatest inward shifts.
They found that the greater the decrease in the resting point of accommodation, the greater the reduction in visual acuity, or keenness, at distance. Changes in the resting point of accommodation did not correlate with subjective eye fatigue.
On the other hand, greater inward shifts in the resting point of vergence were associated with greater eye fatigue, but not with changes in visual acuity.
The more you work at close distances, the more your eyes try to adapt by bringing your resting points closer.
This inward movement of the resting point of vergence could be the visual system's reaction to fatigue.
How Viewing Angle Affects Our Ability To Converge
The eye muscles require less effort to converge when looking down and to diverge when looking up (Bielschowsky 1940). As Krimsky (1948) wrote, "When looking upwards, the eyes tend to diverge...and when they look down, the effort to converge is much easier."
The resting point of vergence changes with gaze angle (Heuer 1989). The lower the gaze angle, the closer the resting point of vergence.
With a 30-degree upward gaze angle, the unstimulated eyes converge on a target about 53 inches away. Looking horizontally, as you would with an eye-level monitor, the resting point of vergence distance is about 45 inches.
When looking down 30 degrees, the resting point equals about 35 inches of viewing distance. Looking down 40 degrees brings the resting point of vergence in to about 32 inches.
Not everyone changes their resting point of vergence when looking up and down. But for those who do, a Downward Gaze™ reduces headaches and eyestrain (Tyrrell 1990).
Limitations on Maximum Viewing Distance
Some guidelines have set limitation on maximum viewing distance. In reality, the only limit to viewing distance is your ability to read the screen. In general the farther away your monitor is from your eyes, the better. But once you have moved it out to your resting point of vergence, moving it farther away will not produce any additional benefits (Ankrum 1996).