On the front of a camera lens there is a glass lens that focuses light into the camera body and onto the film. Inside the lens body, there are several other optical lenses that further refine the image. These lenses are sometimes called "elements".
In front of the first optical lens, there is a small ring with screw threads cut into it. These screw threads allow for filters and other accessories to be easily attached to the front of the lens.
Each lens carries a second mm rating that tells the diameter of this front attachment point.
Each lens has a focusing ring. This is a section of the lens that rotates to allow the photographer to focus the image. On automatic cameras, this ring is moved by a small motor within the lens whenever the photographer presses the shutter release button halfway down. These rings are usually marked with guide numbers showing how far away a subject is when focused.
Focal Length Ring
Each lens that has zoom capability has a focal length ring. This ring allows the photographer to zoom in or zoom out on a subject. Lenses are often described by their focal length. For example, a lens may be called a 70-300mm lens. This indicates that the lens can zoom from 70mm to 300mm. Example of images taken with different focal lengths.
The aperture ring on a lens allows the photographer to control the aperture within the lens.
These settings are marked on the lens using F-Stops. On automatic cameras, the aperture can only be controlled through the camera body F-Stop settings.
The aperture is an adjustable opening in the lens used to allow light onto the film or digital surface. The size of the aperture is measured by the F-Stop setting.
The larger opening of the aperture results in less light needed to expose the image and less depth of field (less in focus). A smaller opening of the aperture results in more light needed to expose the image and more depth of field (more in focus).
The lens mount is a metal area that has been machined into a particular shape to fit a specific camera body type. Each camera manufacturer uses a different lens mount design. The lens is attached to the camera at the lens mount ring by lining up a small dot on both the camera body and the lens. The lens is then gently rotated into place. The lens mount also contains contacts that will match up with contacts on the lens ring mount to allow the camera to control the lens.