Traditional Culture Encyclopedia - Photography and portraiture - What is the zoom principle of telephoto camera?

What is the zoom principle of telephoto camera?

1. The focal length is not the distance between the lens and the negative. In the picture I uploaded, you can see F' (the distance from H' to F' in the picture), and F' is the focal length. Of course, the picture is a simplified model. In fact, a lens has more than ten lens pieces.

2. See/k/548/course/movies/zoomphoto.gif for the principle of zoom lens, but the lens group in the picture you see at the above address is also simplified.