A lens acts as an optical system that generates an image of an object. The simplest form of a lens is made up of a collecting lens, which gathers together rays of light and projects them onto an image plane. Modern camera lenses, however, are made up of multiple lenses and mirrors. Some camera lenses contain as many as 20 lenses that are partially fixed to each other and partially movable. The basic operation of lenses always corresponds to that of a collecting lens. The image produced by the light is depicted upside down. Digital cameras turn this image upright automatically so that the image can be viewed by the photographer as it would be perceived in reality.

A lens is largely defined by two factors: the focal length and the aperture width.

The focal length is given in millimetres and describes the size of the object in correlation to the distance from the lens. In photography, there is a difference between a lens with a fixed focal length and a zoom lens, the latter of which enables the photographer to adjust the focal length within certain limits. When using a zoom lens, it’s possible to change the focal length by shifting individual lenses and thus, focus on a certain object. This capability makes this kind of lens quite versatile. Several advantages of using a lens with a fixed focal length, on the other hand, are its compact design and the perfect optimisation of its components for that certain focal length. The larger the focal length of a lens is, the larger the photographed object will appear at the specified distance. As a result, this reduces the image detail, i.e. the visible content in the image. There are also special wide-angle lenses that allow the photographer to take pictures with an especially high degree of image detail.

The aperture width specifies the size of the opening of the aperture. This determines the amount of light that goes through the open aperture to the lens. The ratio of the focal length to the aperture width is referred to as the exposure time. The f-number is given as either 1/x or f/x. The larger the denominator of the fraction, the less amount of light passes through the aperture. Thus, an f-number of f/11 allow much less light to pass through the aperture than an f-number of f/1.4.

Lenses are categorised according to their different characteristics, which include focal length, angle of view, the type of lens mounts, as well as the individual lenses used in the objective lens. Lenses are complex and extremely sensitive technical apparatuses that function as the “eye” of the camera. Thus, these very important camera components are some of the most expensive parts of the camera. For interchangeable lens cameras, the price for an individual lens often exceeds the price of the camera itself.