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.
For this exercise, I took a couple of shots of a road traffic sign. For these shots I used a Canon 400D (crop sensor) with a 18-55mm kit lens set at ISO 200 and shutter priority 1/160.
Image 1 – 18mm
Image 2 – 55mm
Whilst the road sign looks similar in both images, the backgrounds look very different. The shorter focal length (Image 1 – 18mm) looks much more natural in terms of how the street scene actually looked. Using a longer focal length (Image 2 – 55mm) brings the background much closer to the fore.