Parallel light rays will be focused at a point called the focal point or principal focal point. The distance from the center of the lens to this point is the principal focal length of the lens (for a thin double convex lens). The principal focal length is abbreviated as f.

The focal length is the distance from the optical center of the lens to the image (the film or digital sensor) if focused to an object at infinite distance.

If the image should be focused on something closer than at infinite distance the lens must be moved away from the image, closer to the object. The maximum amount of light a lens can transmit depends on the diameter of the lens and the focal length. The maximum aperture is the focal length divided by the diameter of the lens.

from double focal point to double focal point (no image possible)

from focal point - parallel light rays (no image possible)

parallel light rays - into focal point / burning glass effect (no image possible)

optical image (typically shown only half from center for analysis)

full optical image

real lens system is much more complex, H - H' is used for analysis

real optical lens system

object - lens - image

With photo and video lenses different focal length generates image perspectives with dramatic differences.

using different focal lengths, used from different positions:
long focal length image taken from further distance telephoto lens	medium focal length image taken from medium distance standard lens	short focal length image taken from short distance wide-angle lens
using a zoom lens from the same position, changing the zoom factor:
zoom lens, further	zoom lens, medium	zoom lens, nearer