A lens is one or more pieces of optical material (most commonly glass or plastic) designed to collect and focus all wavelengths of light at a single focal point, to form a sharp image on the focal plane.
When you look at the specifications of a camera you will notice that its lens is made up of elements and groups.
The term element refers to the individual pieces of glass used in the lens. The term group refers to two or more elements joined together to create one unit. So a lens that has seven elements in five groups has seven pieces of glass with two groups of two pieces joined together.
The use of multiple elements allows more optical aberrations to be corrected than is possible with a single element, but by itself the number of elements is no guarantee of quality. Aberrations are optical defects resulting in image distortions such as color “fringes” along boundaries that separate dark and bright parts of the image (chromatic aberration) or straight lines appearing curved (spherical aberration).
