and looking only at the better-focused centre. In the eye, the pupil serves this purpose by constricting while the eye is focused on nearby objects. Small apertures also give an increase in depth of field, allowing a broader range of "in focus" vision. In this way the pupil has a dual purpose for near vision: to reduce spherical aberration and increase depth of field. Accommodation of the lens Changing the curvature of the lens is carried out by the ciliary muscles surrounding the lens; this process is known as "accommodation". Accommodation narrows the inner diameter of the ciliary body, which actually relaxes the fibers of the suspensory ligament attached to the periphery of the lens, and also allows the lens to relax into a more convex, or globular, shape. A more convex lens refracts light more strongly and focuses divergent light rays from near objects onto the retina, allowing closer objects to be brought into better focus. Eye care professionals The human eye contains enough complexity to warrant specialized attention and care beyond the duties of a general practitioner. These specialists, or eye care professionals, serve different functions in different countries. Eye care professionals can have overlap in their patient care privileges. For example, both an ophthalmologist (M.D.) and optometrist (O.D.) are professionals who diagnoses eye disease and can prescribe lenses to correct vision. However, typically only ophthalmologists are licensed to perform surgical procedures. Ophthalmologists may also specialize within a surgical area, such as cornea, cataracts, laser, retina, or oculoplastics. Eye care professionals include: Ocularists Ophthalmologists Optometrists Opticians Orthoptists and Vision Therapists Eye irritation Eye irritation has been defined as "the magnitude of any stinging, scratching, burning, or other irritating sensation from the eye". It is a common problem experienced by people of all ages. Related eye symptoms and signs of irritation are discomfort, dryness, excess tearing, itchiness, grating, foreign body sensation, ocular fatigue, pain, soreness, redness, swollen eyelids, and tiredness, etc. These eye symptoms are reported with intensities from mild to severe. It has been suggested that these eye symptoms are related to different causal mechanisms, and symptoms are related to the particular ocular anatomy involved. Several suspected causal factors in our environment have been studied so far. One hypothesis is that indoor air pollution may cause eye and airway irritation. Eye irritation depends somewhat on destabilization of the outer-eye tear film, i.e. the formation of dry spots on