The crystalline lens is a transparent, biconvex structure in the eye. With the cornea it helps to refract light to be focused on the retina. The lens changes shape. This changes the focal distance of the eye so that it can focus on objects at various distances. This adjustment of the lens is called accommodation. It is similar to the focusing of a photographic camera when it focuses its lenses. The lens is flatter on the inside.
The lens is also called the aquula (Latin, a little stream, dim. of aqua, water) or crystalline lens. In humans, the refractive power of the lens in its natural environment is approximately 18 dioptres. This is about one-third of the eye's total power.