THE human lens has the innate ability to change its focus from a distant target to a near one. This process is known as accommodation. Think of a camera, and how we manually focus the lenses to bring images into sharp focus. The lenses of the eyes work a bit like that, but unlike the camera, we do it subconsciously, without obvious effort. Without this process of accommodation, eyesight as we know it will be very different to what we’re used to.
The range of accommodation is greatest in the young and diminishes with age. For example, a teenager would easily be able to focus to read as near as 7cm whereas a 45-year-old man would not be able to focus nearer than 30cm. A good analogy would be to compare an auto-focus camera that has the ability to focus on near and far objects clearly and a fixed focus camera that has a non-adjustable lens and is unable to focus on objects nearer than six metres clearly.