The lens should be considered a key component of an optical device called a camera. That’s right: not a matrix, but a lens. A photograph is an image, and nothing more than a photographic lens forms this image on a photosensitive material. The matrix only converts the image created by the lens into digital form.
A photographer does not have to be an expert in the field of applied optics, but having some idea of how the lens of your camera works will not only not interfere with your creative growth, but will also help make photography more conscious and manageable.
The main objective of a photographic lens – to collect the light coming from the scene, and focus it on the matrix or film of the camera – can handle an ordinary biconvex lens. However, the image quality in this case will be very mediocre due to the abundance of optical aberrations. To ensure optimal picture quality, additional lenses are introduced into the optical circuit of the lens, correcting the luminous flux, correcting aberrations and giving the lens the required properties. The number of optical elements in modern lenses can in some cases reach two dozen or more. Elements can be combined into groups and together they must act as a single collecting optical system.
In addition to the optical unit, i.e. lens systems arranged in a certain sequence, the lens design also includes a number of auxiliary mechanisms that provide focusing, aperture control, changing the focal length (in zoom lenses), optical stabilization, etc.
Frame, i.e. the lens housing connects all its components together, and also serves to mount the lens to the camera.
Sectional photographic lens
Focal length is the main characteristic not only of a photographic lens, but of any optical system in general.
Focal length is the distance from the optical center of the lens to the plane of the matrix or film. This definition is not entirely correct, but it is understandable even to an inexperienced reader in optics. For those who appreciate the rigor of the wording, I will give a more scientific definition:
The rear focal length of the lens is the distance from the rear main plane to the back focus.
F is the focus; ƒ is the focal length.
Why is the focal length called rear? Because there is also a front focal length that is of no interest to us, indicating the peculiarities of the light rays in the opposite direction, i.e. from the camera. Due to the fact that in photography the course of the rays directed from the object to the camera is important for us, and not vice versa, we will mainly talk about the rear focal length of the lens. In all those cases when I use the phrase “focal length” without any qualifying words, I mean the back focal length.
Perhaps the reader is confused by the terms “back main plane” and “back focus”? I’ll try to explain.
The true path of the rays in the lens, consisting of many lenses, is quite complicated and intricate. However, to simplify the calculations, it is permissible to mentally replace all the lenses of the lens with a single collecting lens whose refractive power corresponds to the refractive power of the lens as a whole. In this case, the action of all the refracting surfaces of the lens is reduced to the action of the principal planes of the imaginary lens. The main plane is the conditional plane, crossing which the rays of light change their direction. There are usually two such planes, because the rays of light going into the camera and the rays coming from the camera will be refracted differently. The main plane characterizing the course of the rays in the forward direction (from the object to the camera) is called the rear main plane. It should be considered the conditional optical center of the lens.
Back focus is the point at which initially parallel rays intersect after passing through the lens. Obviously, to obtain a sharp image of an infinitely distant object, the plane of the matrix or film must coincide with the focal plane, i.e. to cross the optical axis of the lens exactly at the back focus point.
The distance between the main plane and the focus is called the focal length.
As you know, the focal length is measured in millimeters. Based on the relationship between the focal length of the lens and the diagonal of the frame, lenses are usually divided into three conditional groups:
normal lenses whose focal length is approximately equal to the diagonal of the frame;
telephoto lenses whose focal length exceeds the frame diagonal;
short-focus lenses whose focal length is less than the diagonal of the frame.
The image angle, as well as the scale and perspective of the image depend on the focal length. The artistic side of the issue is covered in detail in the article “Focal Length and Perspective”.