A typical lens explained

Hover over a label on the diagram below and the description for that part of the lens will appear. Move the mouse away from the image to make the box disappear.

Filter Thread
Most SLR cameras, some digital and a few compacts have a threaded ring at the front end of the lens. This is used to attach coloured and special effect filters as well as other accessories to alter the light that reaches the film. The diameter of the lens' filter thread is measured in millimetres. Stepping rings can be bought to allow ones size filter to fit on a different size thread. It's better to step up, ie use a filter with a larger thread than the lens, to avoid corners being cut off resulting in dark edges to your pictures.
Focal Length
Indicates the magnification and angle-of-view of a lens. The human eye sees things roughly the same as a 43mm focal length of a lens for a 35mm camera. Anything shorter is classed as a wide-angle, while longer focal lengths are telephoto. Because of the comparatively small size of the CCD in a compact digital camera it has a standard focal length of between 6mm and 8mm while a medium-format camera is around 80mm.
Bayonet Ring
Most lenses have a screw thread ring to attach filters in front of the lens. Some, such as Hasselblad and Sigma, also have a bayonet ring to take a dedicated lens hood or filters. The main advantage is that accessories can be attached quickly and easily, without any danger of cross threading and some can be reversed for compact storage.
Focusing Ring
The ring used to adjust the lens so that the subject appears sharp on the film or CCD. Most cameras now have automatic focus (AF), some have manual focus override (MF) and some have a fixed focus (F) lens that ensures things from about 1.5 meters to infinity are relatively sharp. A few cameras have a power focus (PF), which is a manual method, using a motorised focusing ring. Lenses with a ring that allows a good grip make it easier to adjust and focus manually.
Focusing Scale
Indicates the distance from the film plane to the subject, usually in both feet and meters.
Lens Mount
This indicates what lens mount the camera has. Sometimes older fitting lenses can be added directly to a camera or by using an optional adaptor/converter, but the range of features may be reduced.
Aperture Ring
A ring that is situated near the lens throat on all manual focus lenses and some autofocus models that adjusts to control the amount of light reaching the film or CCD when recording an image. Cameras that don't have an aperture ring often have an electronic control to adjust the aperture either automatically or manually.
Depth Of Field Scale
A scale on the lens that indicates how much of the subject, from the nearest to furthest point from the camera, will be in focus.
Zoom Ring
A zoom lens offers a continuously variable focal length, normally without the need to refocus. A wide-angle zoom covers a range of focal lengths that include a wide angle setting. A standard zoom goes from a slight wide angle to telephoto and a telezoom covers a range of telephoto focal lengths. Some zoom lenses are called super zooms because they cover a larger range of focal lengths from wide angle to longer telephoto.
Although some lenses are sold and boxed by a camera manufacturer they sometimes team up with an optical company to produce a better quality lens. Hasselblad and Contax, for example, use Carl Zeiss optics and that name appears on the lens.
Lens Elements
The individual pieces of glass that form the overall optical construction. Most lenses have between four and eleven elements to bend the light rays to ensure a perfectly formed image appears on the light sensitive surface, such as a film or CCD. The elements are arranged in groups and may be seen in specification sheets as, for example, 6 elements in 4 groups. That configuration may have two single elements and the other four grouped in pairs.
Lens Coating
Transparent coated on lens elements that helps reduce reflections and flare to improve image contrast.

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