What is lens focal length?

What is lens focal length? How to classify the focal length of lens?

Definition of lens focal length:

Lens focal length: refers to the distance from the main point to the focus after lens optics, which is an important performance index of lens. The length of the focal length of the lens determines the imaging size, viewing angle, depth of field and viewing angle of the picture. The focal length of a lens is a very important index of the lens. The focal length of the lens determines the imaging size of the object on the imaging medium (film or CCD, etc.). ), which is equivalent to the proportion of objects and images. When shooting the same subject at the same distance, the image with long lens focal length is large, and the image with short lens focal length is small. According to different uses, the focal length of camera lens varies greatly, from a few millimeters to a dozen millimeters to several meters.

Generally speaking, the focal length we are talking about is the distance from the center of the lens to the focus. But this is only the case of a single thin lens. Because the lens of a camera is composed of many lenses, the situation is far from simple.

The focal length of a lens is divided into image focal length and object focal length. The focal length of an image is the distance from the main surface of the image to the focal point of the image, and the focal length of an object is the distance from the main surface of the object to the focal point of the object. It must be noted that the focal length of the object and the focal length of the image are not necessarily equal, because the telescope structure is widely used in camera lens design, especially zoom lens. What we usually call the focal length of the camera lens refers to the focal length of the image.

Classification of lens focal length:

Common ones are 8mm, 15mm, 24mm, 28mm, 35mm, 50mm, 85mm, 105mm, 135mm, 200mm, 400mm, 1200mm, etc. And it has a super long focal length of 2500 mm.

According to the length of its focal length, that is, the viewing angle when shooting, the lens can be divided into standard lens, wide-angle lens and long focal length lens.

The viewing angle of a standard lens is about 50 degrees, which is the viewing angle that a person can see with one eye without turning his head and eyes. The feeling observed from the standard lens is basically the same as the scenery we usually see. The standard lens focal length of a 35mm camera is mostly 40mm, 50mm or 55mm. The focal length of the standard lens of 120 camera is generally 80mm or 75 mm. The larger the camera frame, the greater the focal length of the standard lens. However, due to the size of imaging medium (CCD or CMOS) of digital camera, the focal length of standard lens is also inconsistent. For convenience and intuition, when talking about DC lens, the so-called equivalent focal length of a 35mm camera is often used, and this equivalence refers to the equivalence in viewing angle. Only the lens of 35mm camera can be compared with other cameras and digital cameras.

Optical focal length

Focal length is a measure of the convergence or divergence of light in an optical system, which refers to the distance of parallel light from the optical center of the lens to the focal point of light convergence [1]. It is also the distance from the center of the lens to the imaging plane such as the backplane or CCD in the camera. An optical system with a short focal length has a better ability to collect light than an optical system with a long focal length. Simply put, the focal length is the distance from the focal point to the vertex of the mirror.

SLR focal length

If you have seen f= in the English manual of the camera, then the number behind it is usually its focal length, that is, the focal length. For example, f=8-24mm, 38- 1 15 mm (equivalent to 35mm), that is to say, the focal length of this camera is 8-24mm, and the oblique viewing angle is equivalent to the focal length of 38-15 mm of a traditional 35mm camera. Generally speaking, the standard lens focal length of a 35mm camera is about 28-70mm, so if the focal length is higher than 70mm, it means that the telescopic effect is supported. If it is less than 28mm, it means that it has wide-angle shooting ability.

Focal range is the extension of focal length, which is usually divided into general shooting distance and close-up distance. The general shooting distance of a camera is usually marked from a certain centimeter to infinity, and the products designed in the recent stage often provide close-range shooting function (macro) to make up for the problem of not focusing in the general shooting mode. Some cameras emphasize the magical ability of supporting 1 cm close-up, which is suitable for shooting fine objects.

Focal length, also known as focal length, is a measure of light convergence or divergence in an optical system, which refers to the distance from the center of the lens to the focal point of light convergence. That is, the distance from the optical center of the lens to the imaging plane such as the backplane, CCD or CMOS in the camera. An optical system with a short focal length has a better ability to collect light than an optical system with a long focal length.

The lens of a camera is a set of lenses. When the light parallel to the main optical axis passes through the lens, it will converge to a point, which is called the focus. The distance from the focal point to the lens center (that is, the optical center) is called the focal length. A lens with a fixed focal length, that is, a fixed focus lens; The lens whose focal length can be adjusted and changed is a zoom lens. (When a beam of light parallel to the main axis of the convex lens passes through the convex lens, it will converge to a point on the other side of the convex lens, which is called the focal point, and the distance from the focal point to the optical center of the convex lens is called the focal length of the convex lens. There is a focal point on each side of the convex lens. )

Optical center: a special point on a lens through which the propagation direction of light remains unchanged. The lens of the camera we use is equivalent to a convex lens, and the film (or the photosensitive device of a digital camera) is near the focal point of the convex lens, or the distance from the film to the optical center of the convex lens is about equal to the focal length of the convex lens.

A convex lens can image. Generally, when a convex lens is used as the lens of a camera, the clearest image formed by it will not fall on the focus, or the distance (image distance) from the clearest image to the optical center is generally not equal to the focal length, but slightly larger than the focal length. The specific distance is related to the distance between the subject and the lens (object distance). The greater the object distance, the smaller the image distance (but always greater than the focal length).

Because when we take pictures, the distance between the object and the camera (lens) is not always the same, such as taking pictures of people. Sometimes we have to shoot the whole body, we will be far away, and we will be very close when we take a bust. That is to say, the image distance is not always fixed, so if you want to get a clear image, you must change the distance from the film to the optical center of the lens with the different object distance. The process of this change is what we usually call focus.

When the camera lens is adjusted to infinity, it is actually a nominal focal length. In design, the distance between the main plane of the lens and the negative or imaging sensor is adjusted to the length of the focal length, and then the image far away from the lens can form a clear image on the negative or sensor. When the lens wants to shoot a close object, the actual focal length of the lens changes. The focal length is usually marked in millimeters (mm), but you can still see that some old lenses are marked in centimeters (cm) or English. The size of the field of view depends on the ratio of lens focal length to film size. Because the most popular film at present is 35mm, the field of view of the lens is often marked according to this specification. The fields of view of standard lens (50mm), wide-angle lens (24mm) and telescope head (500mm) are different. The same is true for digital cameras, whose photoreceptors are smaller than the traditional 35mm film, so they can get the same image with a shorter focal length.