The Zoom Perspective

You may have heard people say that a Wide Angle lens expands space while a Tele Photo lens compresses it... What exactly does that mean? 

Have a look at these 2 pictures:

focal length=17mm

focal length= 55mm

I've tried to keep the shoe(say subject) size the same in both photos by moving away when shooting on the tele end. But what is the big difference between these two?

Its the BACKGROUND, or rather the size of it.

When shooting with a wide angle lens, the background becomes a lot smaller than the foreground(as if when viewed with the naked eyes, they were separated by a large distance- hence the term expansion of space).

On the other hand, when using a tele lens, the relative difference between the size of the background and foreground is much less( as if when viewed with the naked eye, they are close by-> hence compression of space).

This also means that a wide angle lens gives more background than a tele lens.

I think the following diagram is self-explainatory as to why it happens:

So when composing an image, if you want to show a subject in its environment, use a wide angle lens. However, if your purpose is only the subject, a telephoto is preferable, so as to reduce the amount of clutter in the background.

Note: the 'normal' perspective of human vision is said to be equivalent to a  50mm lens on a 35mm film SLR/ full frame DSLR (sensor size same as 35mm film), which is ~30mm on a Canon crop body and ~35mm on a Nikon crop body DSLR.