How to know the focal length when taking a photo

When a SLR camera uses a fixed-focus lens, the focal length of the lens is the focal length when shooting.

When using a zoom lens with a SLR camera, you can look at the focal length scale on the lens body to know what the focal length is when shooting.

For example, the picture below is a 24-105 zoom lens. There are 24, 35, 50, 70, 105 and other focal length scales on the lens body (look inside the red box). When you turn the zoom ring, the white line segment pointed by the red arrow The numerical value facing it is the focal length when shooting.

Extended information:

Understanding of lens focal length:

Generally speaking, we say: focal length is the distance from the center of the lens to the focus. But this is only the case of a single thin lens. Since camera lenses are composed of many lenses, the situation is far from simple.

The focal length of the lens is divided into image-side focal length and object-side focal length. The image-side focal length is the distance from the main surface of the image-side to the focus of the image-side. Similarly, the object-side focal length is the distance from the main surface of the object-side to the focus of the object-side.

It must be noted that due to the design of camera lenses, especially the widely used telescope structure in zoom lenses, the object-side focal length and the image-side focal length are not necessarily equal. The focal length of a camera lens we usually refer to refers to the image-side focal length.

Baidu Encyclopedia - Focal length