Lens Focal Length
Focal Length is the measurement of the optical center 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.
How Focal Length Affects Your Background (free video)
The Basics of Lens Focal Length (free video)
So what does the focal length change?
Let me show you 5 photographs that all have the same exposure, but different focal lengths.
All shots were taken from the exact same spot with the same camera body.
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.