How does the lens inside the human eye work?

The following was written by Bill Evans and published in the Coolum and North Shore News, Ask the Professionals page on Friday, 26th March 2010.

Q: How does the lens inside the human eye work?

A: The crystalline lens of the eye plays an important role in letting us see. Although the cornea (the clear surface on the front of the eye) is responsible for the majority of the eye’s capacity to focus light on the retina, it is the crystalline lens (located behind the coloured iris inside the eye) that changes shape to let us focus on objects at different distances. When we look at objects in the far distance, the crystalline lens stays in a flatter shape, having less optical power. To look at objects up close such as when we read books, the crystalline lens must change to a more curved shape, increasing in power so we can focus up close. This ability to change focus is called accommodation and is controlled by a small ring-shaped muscle around the lens called the ciliary muscle. Accommodation acts like a reflex action, but may also be consciously controlled.

The crystalline lens in a young person is quite pliable and has no trouble changing shape. However it loses its flexibility gradually from the age of ten to the age of sixty, becoming less able to accommodate to focus up close. This gradual reduction of ability of the eye to focus on close objects with age is called presbyopia. For most people it is around the age of forty to fifty when presbyopia starts to interfere with their lifestyle. Indeed, presbyopia is the most common reason for people aged 40 and above to seek an eye examination. Difficulty reading the telephone book or street directory, particularly at night, and needing to push your book further away to read are signs of early presbyopia. For many people treatment is using reading glasses, bifocals or progressive lenses. Glasses won’t help the flexibility of the lens, nor will they hinder it, but they will enable someone with presbyopia to read comfortably.