accommodation

accommodation Optical instruments (such as microscopes, cameras, and some telescopes) work by passing light through the curved surfaces of lenses, where the light is bent (refracted) to bring it to focus at a particular point (e.g. on the film of a camera). Their power depends on the tightness of curvature of these surfaces. To adjust focus for targets at different distances, most optical devices move one or more of their lenses closer to or further from the focusing point. The human eye contains several refractive surfaces, but it alters focus or ‘accommodates’ largely by changing the curvature of one of them — the front surface of the lens inside the eye. The more curved this is, the more powerful the lens becomes, focusing the eye on closer objects. And when the lens flattens, the eye focuses on more distant objects.

This is not the only way of focusing an eye. In chickens, accommodation is produced partly by changing the curvature of the cornea itself. Animals that move between air and water have a special problem, because immersion in water greatly reduces the optical power of the cornea (as anyone who has opened their eyes under water knows). Diving birds gain the extra focusing power that they need under water by squeezing the lens forward into the aperture of the very tough iris, which greatly increases the curvature of the front surface.

The lens of the human eye is a stiff gel of transparent protein, inside an elastic capsule. It is held in place by fine, elastic zonular fibres, which run from the equator of the lens to the muscular ciliary body, which runs around the inner wall of the eyeball, just behind the iris. Accommodation is controlled by the ciliary muscle inside the ciliary body, which encircles the lens. This muscle, which is of the ‘smooth’ type, is controlled by fibres of the parasympathetic division of the autonomic nervous system. When the ciliary muscle contracts, it does not squeeze the lens directly but reduces the tension of the zonular fibres, which relaxes the tension on the capsule of the lens, causing it to assume a more spherical shape, hence increasing the curvature of its front surface.

Accommodation can be prevented with eye drops containing the drug atropine (belladonna — originally extracted from the plant deadly nightshade), which blocks the contraction of the ciliary muscle. The nerve cells that command accommodation (and pupil constriction) are in a small structure in the mid brain called the Edinger–Westphal nucleus, close to many of the cells that direct eye movements.

With the exception of the first few months of life, when accommodation is not fully functional, accommodation becomes progressively less effective with age, with the ability to change lens power falling to zero at an age of about 50 years. This loss of accommodation, or presbyopia, is believed to be caused by changes in the stiffness of the lens proteins, rather than changes in ciliary muscle strength, or in lens size, which increases throughout life. With its accommodation frozen, the eye becomes like a cheap fixed-focus camera, which relies on the depth of field produced by a small aperture to give reasonable focus at different distances. Older people have most difficulty reading fine print (because they have to hold it far away to focus it), especially in dim illumination, when the pupil dilates and the depth of field decreases. Interestingly, people becoming presbyotic do not usually complain that things actually look blurred when held too close, but that what should be black and white looks grey.

The optical quality of the human eye is far from perfect. The great German physicist, Hermann von Helmholtz, once remarked that if he were offered an instrument with such defects by a manufacturer he would send it back — but that he was gladly hanging on to his own eyes, with all their shortcomings! One striking defect, or ‘aberration’, of the eye is that it does not focus light of different wavelengths in the same plane. If deep red is in sharp focus, violet will be somewhat de-focused, and vice versa. Interestingly enough, the retinal cones that are most sensitive to blue– violet light are very few and far between compared with those that are most sensitive in the yellow–green part of the spectrum (the so-called ‘green’ and ‘red’ cones), and are absent in the central fovea (the part of the retina that we point towards things when we look directly at them). Perhaps this is an adaptation to the impossibility of focusing all wavelengths at the same time.

Stuart Judge

See also atropine; autonomic nervous system; colour blindness; eyes; refractive errors; smooth muscle.