One of the main parameters that you need to pay attention to when choosing a lens is the focal length. It is it that determines the viewing angle that the camera will capture when working with one or another optical device. This characteristic is measured in millimeters, centimeters and meters. This parameter indicates how close or far the lens can see. The focal length is the physical value of the lens itself, which does not change. It does not depend on the type of camera with which this optical system is used. Usually the lens size depends on the focal length, that is, the larger the latter, the longer the device will be.
The optical system has the following physical characteristics: back main plane, back and front focal lengths. The first parameter is a plane perpendicular to the lens axis. It can be located both within the device and outside it. It depends on the type of lens and lens shape. The front focal length isa secondary parameter that does not affect the result of photography, but the rear is a very important characteristic. It implies a segment from a sharp image of an object located at an infinite distance from the device to the main rear plane. What does it mean? If the subject is in front of the lens, then the sharp image will be behind the lens. This distance corresponds to the focal length.
Optical systems are usually divided into two types: fixed and variable. A lens whose focal length can be changed is also called a zoom lens and a varifocal lens.
Let's look at examples of how this parameter is indicated on the case of an optical device and what it means. Let's take lenses with a fixed focal length from Nikon, on the body of which 85 mm is written. Such an inscription means that this is a device with a fixed distance of 85 millimeters. For the second example, let's take a Tamron zoom lens. Its body is marked 28-200 mm. This marking means that the focal length of the optical system can vary from 28 to 200 mm.
Every professional photographer usually has a set of lenses that cover the required range of distances. This allows you to cover all situations that may arise during work. Such a set usually includes optical instruments with the following parameters: from 14 to 24 mm, from 24 to 70 mm, from 70 to 200 mm, from 200 to 400 mm.
Shouldbe aware that, in addition to the angle of view, the focal length also affects the perspective of the picture. So, with different indicators, the sizes of the objects being shot will be correlated differently in the images. The long-focus optical system brings not only the object being photographed closer, but also the space in front of and behind the subject of focus. A wide-angle device averages the sizes of all elements in a photograph. Also, different focal lengths affect background blur.
In conclusion, let's say that only by determining the desired range, you can choose the right lens. The focal length significantly affects the blurring of the blur zone, as well as the perspective of the picture.
