As you will see from the diagrams that follow, the purpose of the lens is to present as sharp an image on the retina as possible.
Click on Lens to graphically see what the lens does.
The ciliary muscle helps to change the shape of the lens. This changing of lens shape is called accommodation. If you click on ciliary you can see the anatomical structures involved and how the lens shape change takes place.
The lens itself is a multilayered structure (something like an onion). In young people it is normally perfectly clear and quite elastic. As one ages its elasticity is reduced. In fact after the age of about 45 the lens' ability to change in shape is considerably reduced. That is why people over the age of 45 almost always require glasses to read and/or to see distant objects. It is not unusual for people in their 50's and older to wear bi-focal or even tri-focal lenses.
As one gets older the lens can also become cloudy. This condition is called cataract. When cataracts become too severe the lens has to be removed and be replaced with an artificial lens. Of course the artificial lens is not capable of accommodation. However, by the time most people are afflicted with cataracts they will already be old enough so that they will have naturally lost most of their accommodation. Also as one ages the lens becomes more yellow. People in their 50s will, for example, exhibit clearly lower spectral sensitivities at the short wavelength end of the spectrum than 10 year olds.
The fact that the outside world is imaged upside down on the retina used to baffle the early philosophers.
To see an illustration of this click on INVERTED IMAGE.