Select your longest focal length and compose a portrait shot fairly tightly within the frame in front of a background with depth. Take one photograph. Then walk towards your subject while zooming out to your shortest focal length. Take care to frame the subject in precisely the same way in the viewfinder and take a second shot.
Having resorted to bribing my toddler with a biscuit, and still having to restrain her in the buggy, I had a go at this exercise! I didn’t really get the results I was hoping for as she was in the buggy so the back of the chair formed a large part of the background. In Fig1 I was at maximum zoom (240mm) but I found as I tried to frame the image the same as I walked closer to take fig 2 (24mm) I was closer than the minimal focal length. I stepped back to take Fig 3 also at 24mm, but this time (almost) in focus, and now much more of the background is apparent.
Although I wasn’t able to recreate the same framing in this instance, I have already written about the effects of different focal lengths on perspective distortion on an earlier blog post when looking into which lens to hire for a job.