Camera lenses are made up of not just one single lens but many different lens ‘elements’, sometimes cemented or fixed together in ‘groups’. A comparatively simple prime (non zoom) lens may have 6-7 elements while a complex zoom lens might have 17 or more. The different lens elements are needed to compensate for a variety of common lens aberrations and offer autofocus and zoom capabilities.
The lens is a fundamental part of any camera. It's what creates the image on the camera sensor (or film). Some cameras have a fixed, non-removable lens while others offer interchangeable lenses. Your choice of lens has a major impact on the appearance of your pictures, including the lens's focal length (angle of view) and its aperture setting (which you may or may not be able to adjust). At a simple level the lens is just the thing on the front of the camera, but on a more advanced level lenses open up a whole world of photographic choices, buying decisions and technical comparisons.
Here's a selection of further lens-related topics
Aspherical lenses offer better correction for many common lens aberrations than regular spherical lenses. Lenses with a spherical profile are easier to grind into the correct shape, but aspherical lenses have a more complex profile that’s more difficult and more expensive to make. Many modern lenses use moulded aspherical elements instead to get round this. Lenses with aspherical elements in their design command a higher price and generally give better results.
Reproduction ratio is a term used in macro photography to indicate the degree of magnification. A ratio of 1:1 is usually considered the minimum for ‘true’ macro photography. This means that an object is reproduced at exactly the same size on the sensor or film surface as it is in real life. If the first number is higher, e.g. the reproduction ratio is 2:1, it means the lens can reproduce objects at twice life size. If the second number is larger, e.g. 1:2, it means objects are reproduced at half their actual size.
An apochromatic (APO) lens is designed to offer improved correction of chromatic aberration and spherical aberration using specialised materials and combinations of lens elements. It’s a selling point for lenses, though only indicates the lens design used and isn’t really a guarantee of good performance on its own.
A system introduced by Nikon for some of its lenses where the lens aperture diaphragm in the lens is controlled electromagnetically rather than by the traditional mechanical linking. This gives more accurate and consistent exposures, especially during continuous shooting, where the lens diaphragm may be adjusted many times a second.