This article is from the Birds FAQ, by Lanny Chambers with numerous contributions by others.
For both these questions, see the Optics FAQ, posted regularly in
rec.birds by Ed Matthews <firstname.lastname@example.org>.
The Optics FAQ is archived together with this and many other FAQs.
See the question "How can I get this and other FAQs by anonymous FTP?
On the Web?" in the other part of the FAQ.