People often confuse the terms "zoom lens" and "telephoto lens". Lenses have focal length. Telephoto lenses have a longer than normal focal length. Wide angle lenses have one shorter than normal. A zoom lens is one that has an adjustable focal length. For our purposes right now, focal length is not an important concept. The field of view of a telephoto lens has a smaller angle and it has a higher magnification than a normal lens.
That last shot is a nice segue to the topic of zoom.
We usually think of using a telephoto lens to make the subject larger when we can't get close enough. But the length of a lens has a more subtle effect that can be quite powerful: A telephoto lens compresses the depth of a photograph, and a wide angle lens exagerrates it. Let me try to illustrate this.
An obvious difference is that the far tower seems to be a lot larger, or closer when viewed with a telephoto lens. Yet the foreground cars aren't enormous. The telephoto lens didn't magnify the close objects as much as it did the distanct objects. So the cars way in the back are closer in size to the cars up front. You brain thinks that the distant cars must be closer in distance to the cars in front of them, so depth of the photo seems compressed. The traffic in the second photo therefore looks heavier.
It is a subtle effect, but it's used all the time. Have you seen a movie where the hero runs toward the camera while the building behind him explodes? Obviously, the studio doesn't want the star in serious danger, so they don't want him too close to the building when they set off the fireworks. To make him look like he's close, the camera is outfitted with a telephoto lens so the exploding building in the background looks much closer to the actor.
Portrait photographers use telephoto lenses for several reasons. Remember get closer? People have a personal space, and they can grow uncomfortable when strangers get too close, especially strangers with cameras. Thus a slightly telephoto lens helps a portrait photographer get a little closer without actually getting closer. A telephoto lens generally has a smaller depth of field, which makes it easier to throw the background into soft focus (minimizing distracting details and emphasizing the subject), even when there's more than enough light. Finally, although it's a subtle effect, the telephoto lens compresses depth, which flattens people's faces, which is generally flattering. Consider the alternative: a wide angle lens exagerrates depth. If a portrait photographer used one of those and got close the her subject, the model's nose would be huge compared to the eyes and ears. A slighty telephoto lens has the opposite effect.People