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. Compare the two images and make notes in your learning log.
With the longer focal length (70mm), the background is more blurred. However, with the shorter focal length (24mm), there is a greater sense of depth.
The two different focal lengths show completely different elements in the frame.