What is the image-side focal length and object-side focal length?

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.

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 widespread use of telescope structures in zoom lenses, the object-side focal length and image-side focal length are not necessarily equal.