When we look back and forth between objects at different distances, the ciliary muscle in the eye changes the shape of the lens capsule (Kaufman 1992). The light rays bend so that they strike the retina at a single point. This allows a sharp image to be interpreted by the brain.
If that point is too far in front of or behind the retina, blurring results. In reaction to this blurring, the brain causes the ciliary muscle to change the shape of the lens capsule. That brings the object into focus. The closer the object, the more the lens must accommodate.
Accomodation And Distance
When our eyes relax and have nothing specific to look at, they automatically focus at a distance called the resting point of accommodation.
Not so long ago, scientists believed that the resting point of accommodation was infinity. They recommended that we "look off into the distance" to relax our eyes. Recent studies show that the resting point of accommodation is not infinity, it's much closer (Owens 1984). While it differs among individuals, it averages about 31 1/2 inches (Krueger 1984).
When you work at distances closer than your resting point of accommodation, your ciliary muscle must work harder to focus. This contributes to eye strain, (Jaschinski-Kruza 1988). The closer the object, the greater the strain. The ciliary muscle must work two-and-a-half times harder to focus on a monitor 12 inches away than it does to focus on one at 30 inches (Fisher 1977).
How Viewing Angle Affects Our Ability To Accomodate
When we change our viewing angle, we also change our eyes' ability to accommodate. Ripple (1952) found that subjects over age 42 increased their ability to accommodate by an average of 25 1/2 percent by directing their eyes down in the "usual reading position." The improvement for younger subjects was even greater.
Try this for yourself: Hold a business card with text on it at arm's length in front of your eyes. Bring it toward you until the letters just begin to blur. Without moving your head, gradually lower the card in an arc, keeping it the same distance from your eyes. You should notice the letters getting sharper.