What is the focal length of the camera?

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. 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.

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)" means that the focal length of this camera is 8-24mm, and the diagonal viewing angle is equivalent to 38- 1 15mm of the 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 as "from a certain centimeter to infinity", and products designed by advanced technology often provide close-range shooting function (macro) to make up for the problem of not focusing in general shooting mode. Some cameras emphasize the magical ability of supporting 1 cm close-up, which is suitable for shooting fine objects.

The lens of a camera is a set of lenses. When parallel light passes through a lens, it will converge to a point, which is called the focus. The distance from the focal point to the center of the lens 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 parallel light passes through the convex lens with the main axis of the convex lens, it will converge to a point on the other side of the convex lens, which is called the focus, and the distance from the focus 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: the center of the convex lens can be approximately regarded as the optical center.

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 from the clearest image to the optical center (image distance) 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".