Focal Length is the measurement of the end of the lens to the film plane. Lenses come in different “focal lengths” and more and more you see mostly zoom lenses with a variable focal length (ie 18-55mm). A full extended zoom lens in this examples would be 55mm. Zoomed down the lens is 18mm.

So what does the focal length change?

Let me show you 5 photographs that all have the same exposure, but different focal lengths.

28mm

50mm (normal)

100mm

200mm

400mm

I simply doubled the focal length with each photo and used the same exposure (f16 @ 1/500 using Fujicolor 400). As you can see the longer the focal length the closer to the action we get. Also, as we get closer to the action and the focal length is longer there is a more narrow depth of field (background is more blurry). Ever wonder why that happens? Find out the physics of depth of field.