What is the focal length in photography?

Lead: Focal length is a basic concept in photography. Whether you are conscious or not, you will use the focal length every time you shoot. So what is the focal length? What's the effect on the photos?

Focal length in photography

Focal length is an optical term, which refers to the distance from the center of the lens to the focus. A photographic lens is composed of lenses, so it also has a focal length, that is, the distance from the main point to the focus after lens optics.

Focal length is a basic index of lens and one of the most important indexes. We often use focal length to describe or name a lens, such as 50mm lens and 35mm lens.

Basic function of focal length

In photography, the essential function of focal length is to determine the shooting angle of the lens, that is, the range of pictures that can be shot. The shooting angle is actually determined by the focal length of the lens and the size of the film or sensor, but it is customarily based on the most common 35mm camera.

The above picture shows the relationship between focal length and viewing angle (35mm). As can be seen from the figure, the viewing angle of the 15mm lens is 180 degrees, that is, the picture in the range of 180 degrees in front of the lens can be photographed. The angle of view of the 50mm lens is 46 degrees, which is basically close to the angle of view of the human eye, so the picture taken by the 50mm lens is closest to what the human eye sees, so it is also called the standard lens.

It needs to be emphasized again that the correspondence in this figure is based on a 35mm camera. If other frame cameras are used, the corresponding relationship will also change. Traditionally, we will use the focal length multiplier to convert the actual focal length into an equivalent focal length of 35mm. For example, if the focal length magnification of Canon APS-C camera and 35mm camera is 1.6, then the viewing angle of installing a 50mm lens on Canon camera in APS-C format is equivalent to 80mm on 35mm camera.

Lenses are classified by focal length.

Traditionally, based on 35mm cameras, we classified the lenses into several categories according to the focal length. The three basic categories are wide angle, standard and telephoto. On this basis, it can also be subdivided into fisheye, ultra wide angle, medium telephoto and ultra telephoto.

Specifically, generally, the lens below 35mm is called a wide-angle lens, the lens above 80mm is a telephoto lens, and the middle part is called a standard lens. Because different focal lengths have their unique imaging characteristics, this classification is helpful for everyone to choose lenses.

Selection of lens focal length

Due to different viewing angles, the pictures taken by lenses with different focal lengths will be different. The standard lens is closest to the human perspective, and the shot picture is closer to the actual scene we see. Therefore, the standard lens is suitable for almost all subjects, such as special news or documentaries. But it is precisely because it is very close to what the human eye sees that it is more difficult to take distinctive photos.

Wide-angle lens has a larger angle of view, so the picture will show a wider range that the human eye can see, which is generally suitable for taking photos showing large scenes such as scenery. In addition, because the wide-angle lens has a strong sense of spatial perspective, it can make photos more visually impactful, so it is very popular with documentary photographers.

The telephoto lens is actually a telescope, and the shot scene is smaller than what the human eye can see, which is conducive to achieving the shallow depth of field effect, so it is very suitable for taking photos with clear subjects, especially portrait photography or wildlife photography. For example, 85- 135mm is called the golden focal length of portrait photography, because it is easy to take photos with blurred background and prominent people. Because the telephoto lens sees far? Function is almost indispensable in sports, wildlife and other disciplines, and even some astronomy enthusiasts will connect cameras to astronomical telescopes to shoot stars such as the moon.