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.
For this exercise I took photos of my husband at the top of our lane so that we have the depth behind us. I found this quite difficult to ensure that my husband was precisely the same in the viewfinder. So I do think this might of been better to do with a tripod to ensure that the camera was kept exactly still. As you can see I uploaded another two as I wanted to practise this a bit more so I tried again in my garden. Its amazing how your focal length can change so much in the background. I found it made it seem like I was taking photos looking uphill when I zoomed out and walked towards Matt. It allowed a lot more into the background and even made it look like I had changed my angle.
Heather Wade OCA Blog 