Georgia State University
Georgia State University: Hyper Physics: Telephoto Lens
At this site from Georgia State University Physics Department the operation of a telephoto lens is discussed. The refraction of light through the lens is shown and a link to a calculation page is provided.
Other
Mas: Lunar Photography With Telephoto Lenses
Ever taken a picture of a nice large full Moon, only to find that it looks like a small dot on your finished photograph? This site explains why, and offers tips on taking good pictures of the Moon.