A telephoto lens is a lens whose focal length is longer than normal focal length (for the sensor size) and therefore has a magnifying power on distant objects, just like binoculars or telescopes.
Here is an example of a telephoto lens:
Click to enlarge: Canon telephoto lens. Photo © Jan Kuchenbecker via Wikimedia Commons
A bit of history
The first telephoto lens in history was actually the first telescope. It was invented by a Dutch optician whose name escapes me. Then an Italian astronomer and scientist who lived in the 17th century, Galileo Galilei, built a telescope of his own and was the first to observe the Earth’s Moon in details. He saw mountains there and became convinced that it was an other world in it’s own right. Then he discovered the four moons of Jupiter, thereby starting a new era for the science of astronomy. This greatly displeased the powerful Catholic church! Galileo was forced to recuse himself, i.e. to deny his own discoveries, or face the pain of being sent to the stake! Ouch!
Compression of perspective
The telephoto lens is therefore intended to magnify distant objects. It also has the side effect of compressing the perspective and make seem to be close to each other, objects that are actually quite far from one another.
Here’s an example of spatial compression due to the telephoto lens:
Click to enlarge: Walk on Mont-Royal in Montreal with telephoto compression – Photo © Charles Martel 2002
As you see, the people walking in the foreground seem to be literally pushed onto the buildings behind. And the small mountain in the background also seems to be close to the buildings, while it is actually on the other shore of the St. Lawrence River and several tens of kilometers away!
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© Charles Martel 2015