As I have just acquired a long lens a 70-300mm I thought I would try it out and at the same time show the difference between zooming with your feet and zooming with a lens.

When shooting a subject from a fixed position with different focal length lenses all that changes is the field of view the actual image stays the same in that the perspective in the image is exactly the same so if you cropped so that the subject filled the frame then the images would look just the same except the ones that have been more severely cropped will have less pixels in them.

If you fill the frame when using different focal length lenses by altering your position (zooming with your feet) and getting closer with a wide angle and further away with a telephoto then the image perspective will change.

The following three images of Hardwick hall are taken from the exact same position with focal lengths of 11mm, 100mm and 300mm and the castle is the same in each (the images are uncropped apart from a little straightening )

Taken at 11mm

Taken at 100mm

Taken at 300mm

Here are the  images from above cropped to give the same framing

Now the images of the bench below have been taken with similar settings but I moved in closer each time to fill the frame which changes the perspective (now you can see why a wide angle lens is unflattering in portrait work )

300mm

70mm

11mm