What effect does the focal length of the lens have on taking pictures?

1, when the parallel beam is incident, the focal length is from the transparent mirror to the light focusing point.

2. The shorter the focal length, the wider the field of view and the wider the framing range, and the smaller the area occupied by the scene in the picture.

3. The longer the focal length, the narrower the field of view, but the larger the area of each scene in the picture, so you can increase the zoom shooting when shooting the distant view or close-up.