What is the focus and what is the focal length?

Focal length is originally a quantity in optics. When a beam of parallel light passes through the convex lens with the main axis of the convex lens, it will be converged into 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: 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 are far away, and we take a bust, and we are close. 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".

From the above description, the focal length of the convex lens is a fixed number. In other words, the focal length of the convex lens is not adjustable. The focal length of a considerable number of cameras can not be adjusted. This kind of lens is called "(fixed) fixed focus (far) lens". So the "focus" mentioned above (we usually) doesn't really adjust the focal length of the lens! Just another part of the camera, its lens is "zoom lens", that is, those lenses that can be "zoomed". Zooming with such a camera is the real "focusing"!