Many will say that focal length also determines the perspective of an image, but strictly speaking, perspective only changes with one's location relative to their subject. If one tries to achieve the same subjects filling the frame with both a wide angle and telephoto lens, then perspective does indeed change because one is forced to move closer or further from their subject. For these scenarios only, the wide angle lens exaggerates or stretches perspective, whereas the telephoto lens compresses or flattens perspective.