The way we see objects in an image is dependent on the distance it is removed from us and the type of lens used. If a subject is captured with a 50mm lens (35mm equivalent) we experience a more natural perspective than if it is captured with a wide-angle or tele lens. With a 50mm lens or at a standard zoom setting we can guess the difference between objects according to our real life experience.
Wide-angles open up perspective, so it is much more difficult to guess the actual distance between subjects. Our view will be lead astray by the way these lenses open up perspective, and objects nearby appear to be much wider apart than those farther away. On top of this the background seems to move far away into the distance.
Tele lenses compress perspective and objects appear to be closer together than in real life. The background is drawn towards the foreground and seems to be a lot closer to the main subject than we actually see it with our own eyes. The longer the focal length the more obvious this effect is.
Since tele lenses have shallow depth of field even at smaller apertures, the background may be blurry ( which can be effectively used to separate subjects from their background) but it seems to be relatively close. In this case we speak of "flat" perspective, .