[Mika]

This recently caught my attention:

http://en.wikipedia.org/wiki/Visby_lenses
http://www.frojel.com/Documents/Document03.html
http://www.leinroden.de/22visby.pdf

And yes I assume that people reading this forum are able to read German languages as their second nature. If not, there will be at least pretty pictures involved.

Given the lenses were done 1000 years ago by Vikings, they are quite an achievement, which also tells about their understanding of optics. It must have been much better than currently assumed.

With focal lengths of 22 mm and diameters of 50 mm, the numerical aperture will be around 0.7, which translates to the F/# of (1/2*NA) = 0.71! If the spot size (I assume this is on-axis spot size) is 25 to 30 µm! That's pretty badass lens, even by today's standards!
I would be tempted to go and measure the index of refraction of that glass, and the Abbe number should it matter! But I suspect that the lens exhibits a large amount of chromatic aberration, unless they were smart enough to figure out a way to achromatize it back then...

And come to think there are people who are worried about NASA's (unconfirmed) ability of not being able to reproduce the heat withstanding tiles in front of the shuttle - which could be hazardous materials related stuff actually, nowadays you simply cannot put anything on those parts...

Mika

[Nuke]

its what happens when chistians, who are incapable of reading, go around mindlessly burning books. vikings rule!