Lens Matters This section is about focal length: how a wide angle differs from a telephoto, and everything in between. This is one of THE essential topics to understand about photography.
The Lens Field of View Sequence This sequence seen in lens catalogs has a lot to teach us about focal length; as a bonus it will convince you that you need to buy more lenses.
The Lakeside Sequence We keep Nicole's head the same size, but change focal length and distance. See how telephoto narrows our view of the background while the wide-angle shows lots of background.
High-Speed Lenses In the old (film) days fast lenses were critical for shooting in low light. With today's high ISO they are used more for the narrow focus look.