Find a scene that has depth. From a fixed position, take a sequence of five or six shots at different focal lengths without changing your viewpoint. (You might like to use the specific focal lengths indicated on the lens barrel.)
As you page through the shots on the preview screen it almost feels as though you’re moving through the scene. So the ability to change focal lengths has an obvious use: rather than physically moving towards or away from your subject, the lens can do it for you. The other immediate difference between the shots is the ‘angle of view’, which also depends on the sensor size of your camera. Use the sequence to try to get a feeling for how the angle of view corresponds to the different focal lengths for your particular camera and lens combination. Which shot in the sequence feels closest to the angle of view of your normal vision.
For this exercise I used a Canon 40D camera with my 24-105 lens attached and I used focal points close to those marked on the barrel of the lens giving four photos.
The four photos clearly show a difference in using different focal lengths with much less of the overall scene being displayed in the last image which loses the depth of the walls on each side but shows the pot plants and cottage at the end whereas the first photo includes the wheelie bins, the length of the alleyway as well as the plants and cottage.
For me I wasn’t sure whether the 47 mm or 67 mm most the most pleasing shot but in the end felt the 47 had the edge as it showed front to back detail, incorporated most of the elements and was framed by the walls and wheelie bins to the right.
The zooming in didn’t change the appearance of things but did exclude some things from the shot.