“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.”
1
18mm (on crop sensor)
55mm (on crop sensor)
The 2 shot are to all intents and purposes are identical except the focal length used.
The the shot a 18mm you can see the distortion caused by the lens exaggerating the the nose for example, plus you can see more of the background it has more depth.
The shot a 55mm the proportions are much more realistic and normalise, and the background is more compressed – much more complementary to a portrait