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.
Both of these images were taken with a Canon EOS 5D mark IV full frame camera with a 24-70mm lens, set to F/5.6, 1/125 and ISO 3200. The first image was taken with a focal length of 70mm and the second with a focal length of 24mm.
As you can see there is a noticeable difference between the two images, the first image is a much more realistic one than the second image. When you see the two photographs side by side it is really surprising that when zoomed out to the shortest focal length it caused such distortion to the second image, the curvature and bulbousness of the subject and the lines are considerable.
Jo's Photo Blog 