A lens gathers and focuses light. The quality of your images depends more on the quality of the lens than on any other camera component. The most important part of a camera is its lens since the quality of an image is so dependent upon it. The most basic camera body fitted with a good lens can make a good picture, but the best camera body in the world cannot make a good picture if its lens is of poor quality - if the image or parts of it are not in focus or if an inadequate amount of light for proper exposure reaches the film . Any lens, even the simplest one, gathers and focuses light .
Light strikes the front surface of the lens and passes through the glass element.
A photographic lens is really not a lens at all, although it is called one anyway. However, common usage has won in this case. A photographic lens will always be called a lens, even though it's not a lens but has a lot of lenses in it. With the exception of the tidbit of information provided in this paragraph, we refer to the photographic objective as a lens throughout the site. Whenever possible, purchase the best lens in its class.