Accommodation

The eye's ability to change focus between objects at different distances, achieved by the ciliary muscle reshaping the crystalline lens.

Accommodation is what lets you switch focus from this screen to the wall across the room without conscious effort — most of the time. Sustained near accommodation fatigues the ciliary muscle within 20 or so minutes; the 20-20-20 rule's remedy is to force a brief far-focus episode so the muscle can relax. With age (presbyopia), accommodative range diminishes, which is why reading glasses become necessary in the forties. Disorders of accommodation include accommodative spasm and accommodative insufficiency, both treatable with a comprehensive eye exam.