What's the difference between changing the focal length and the object distance when shooting the same scene?

There is a difference, because you look at the problem from different angles. Move the camera closer to the main body, then use a shorter focal length and enlarge the sense of space before and after. At this time, the background range is larger. When the subject is brought closer with telephoto, the sense of space before and after is compressed, and the range of background in the picture is relatively small, which is the change of perspective. In addition, if you are too close to the subject, you have to use a wide angle and there will be deformation. For example, if you pat a person's face, your nose will be bigger than normal.