This is the 2007 version. Click here for the 2017 chapter 04 table of contents.

The Lens and Visual Accommodation

Immediately behind the iris is the lens. Shaped like a magnifying glass, it refracts light even more than the cornea and aqueous humor. It plays a primary role in focusing the visual image on the back of the eye. The lens is not rigid; it can be bulged and flattened somewhat. The lens changes its shape when pulled by fibers called the suspensory ligaments attached to the ciliary muscles. You strain your ciliary muscles most by focusing on near objects, which requires bulging the lens. You relax your ciliary muscles when looking at something far away.

What is the primary role of the lens? Why is it a strain to focus on close objects? What is visual accommodation?
How do glasses and contact lenses work? Why do bi-focals become necessary?

This adjustment of lens shape to shift focus between near and far objects is called visual accommodation (a-com-o-DAY-tion). Researchers have found that even when people imagine looking at faraway objects, their ciliary muscles relax, showing visual accommodation.

Near-sightedness (myopia ) and far-sightedness (hyperopia or hypermetropia) are common problems of vision occuring when the image from the environment is not focused correctly on the back of the eye. The line labeled focal plane in the following figure is the place where the image is clear, not blurry. Glasses and contact lenses change the refraction slightly to align the focal plane with the back of the eye. That is why they are called corrective lenses.

After age 40 the lens becomes less flexible. Bi-focals (lenses with different focal length on top and bottom) may become necessary, to focus on near and far objects. This is called presbyopia (old eyes).

Corrective lenses are necessary if the eye is too long or too short so that the focal plane misses the retina.

After light passes through the lens, it goes through a large chamber in the eye that contains a gelatin-like substance called the vitreous humor. Once light passes through the vitreous humor, it lands on the back of the eye, the retina (Latin for net).

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