What does focal length mean?

Focal length refers to the distance from the center of the lens to the focusing point.

Focal length, also known as focal length, is a measure of the convergence or divergence of light in an optical system. 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.

For a thin lens in air, the focal length is the distance from the center of the lens to the main focus. For a converging lens (such as a convex lens), the focal length is positive, and a beam of parallel light will converge at one point. For divergent lenses (such as concave lenses), the focal length is negative, and parallel beams will spread out after passing through the lens.

The focal length is usually marked in millimeters (mm), but you can still see that some old lenses are marked in centimeters (cm) or inches. The size of the field of view depends on the ratio of lens focal length to film size. Because the most popular specification now is 35mm, the field of view of the lens is often marked according to this specification.

The concept of focal length

A group of lenses with curvature (convex or concave) on both sides or one side are installed in the metal tube of a camera or projector to form a comprehensive lens. The light emitted from different parts of the object, after passing through the lens, is focused on a point on the negative, so that the image has a clear outline and real texture. This point is called the focus. The so-called focal length is the distance from the point in the lens of the lens to the point where the light can be clearly focused.

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".