What is focal length?

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.

Application of focal length in photography

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 distant object can form a clear image on the negative or sensor. When the lens wants to shoot a near object, the distance between the main plane of the lens and the imaging detector or film changes, so that the object with limited distance can also be clearly imaged.

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 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.