Focal length of the lens as one of the most important characteristics of photoptics

The focal length of the lens is the distance between the rear main point of the lens and the back focus of the optical system.

Apparently, the academic definition, from the point of viewview of a simple man, somewhat confused. Therefore, it is important for an amateur photographer not so much to know the definition of such an unquestionably important characteristic as the focal length of the lens, how much to imagine what this characteristic affects. And it affects the approach / removal of the subject in the viewfinder / on the screen / in the picture, as well as for the future.

There are lenses with variable focal lengthdistance ("zoom") and lenses with a constant focal length ("fixes"). In this case, the "zoom" is divided into zoom lenses and zoom lenses. Zoom lenses are usually used in cameras with interchangeable optics, as well as "pseudo mirrors" (cameras, the principle of focusing which is similar to the usual soap dishes, but the controls, and the appearance itself is suitable for mirror cameras). The optical scheme of such lenses is such that it makes it possible to focus on the entire range of focal lengths declared by the manufacturer for this device. Vario lenses do not possess such a useful property and allow the owner to shoot only on a certain set of focal lengths. At the same time, they are more compact, simpler and cheaper than zoom lenses. But most users of such "glasses" do not even suspect such a restriction. Virtually all "soapboxes" of the lower and middle price range are equipped with zoom lenses.

The focal length of the lens, as a rule, is eitheris present in its name (for interchangeable optics), or with small figures and letters just put on it (typical for "soap trays"). In the latter case, it is very important not to get caught in a mess; some manufacturers indicate only the effective focal length of the lens, some only real, and some - both. After the advent of digital cameras, the term "crop factor" entered into the everyday life of photographers. This is the ratio of the linear dimensions of the camera array to the linear dimensions of the standard narrow-film frame (36 mm x 24 mm). All matrices of digital compacts are "sprinkled", i.e. their dimensions are much smaller than the above linear parameters of the film. Therefore, to determine the so-called effective focal length (such as if you are shooting an ordinary narrow-film "soap box"), the real focal length of the lens should be multiplied by the crop factor of the matrix. Sometimes this parameter is also called focal length in the 35-millimeter equivalent. The size of the matrix of the digital camera can be found from the documentation accompanying the camera, or on the manufacturer's website. Most likely, the size will look unusual, somehow - 1 / 2.5 ". Next on the Internet you can find the tables for determining the croping factor for the most common matrix formats.

In terms of the size of the focal length (as a rule, real, and not effective), lenses are divided into the following categories:

ultra-wide-angle (less than 20 mm). As a rule, they serve for shooting panoramic landscapes, architecture;
wide-angle (24-35 mm). Their destiny - shooting in the premises, especially small ones, landscapes, group portraits;
normal (35-70 mm). The human angle of view is equivalent to a focal length of approximately 50 mm. Therefore, this category of lenses is most often used for everyday shooting. Photos taken at these focal lengths look natural;
long-focus (70-135 mm). Such a focal length is possessed by most of the objectives for macro photography, as well as the so-called "portrait" (lenses intended for portrait photography);
telephoto lenses (over 135 mm). Everything is clear - shooting objects that are very far from the photographer. Such a lens will be indispensable when shooting sports events, concerts. Or, say, in photo hunting on wild animals.

Of course, this division of lenses in sizefocal length is very arbitrary, but when choosing optics for a camera, even an amateur photographer must take into account what he plans to do with this glass.