Camera lenses vary greatly in many respects, from the length and zoom capabilities to the extra functions like image stabilization. However, every camera lens shares the same basic structure and elements. Understanding each of these pieces can help you get to know how your photography equipment works and what to look for when shopping.
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 threads allow for filters and other accessories to be easily attached to the front of the lens.
Each lens carries a second millimeter ("mm"—e.g., 49mm to 77mm) rating that tells the diameter of this front attachment point. You will need to shop for filters, lens caps, and other lens accessories using the size designated for that particular lens.
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 you press the shutter release button halfway down. For manual focus, you will turn the ring until the image comes into focus.
The focus ring is usually marked with guide numbers showing how far away a subject is when it is focused. You will also notice f/stops on this scale, that tells you the full range of the focusing distance. In other words, objects within that range will fall into focus using that particular f/stop.
Focal Length Ring
Each lens that has zoom capability has a focal length ring. This ring allows you 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 anywhere from 70mm to 300mm.
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's f/stop settings.
Aperture refers to 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.
A larger opening of the aperture (e.g., f/2.8, f/3.5, f/4.0) results in less light needed to expose the image and shallower depth of field (less of your image is in focus). A smaller opening of the aperture (e.g., f/11, f/16, f/22) results in more light needed to expose the image and wider depth of field (more is 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. Some have changed lens mounts over the decades so older manual camera lenses will not fit on newer SLR and DSLR cameras. When buying lenses, you must ensure the mount matches your camera.
The lens is attached to the camera at the lens mount ring by lining up small dots on 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.