What are the grades of Optical Glass?

No expensive glasstypes, flavio. Not even a Lanthanum. It's an honest performer, all-spherical and *very* cheap to fabricate. Takes 5 minutes to assemble. It is very simply not an eyepiece design form that I've seen anywhere else. It's very interesting how it corrects, but unfortunately I cannot share the details. At some point I'll look into writing a white paper then you will be able to see.

Nodda Duma said:

At some point I'll look into writing a white paper then you will be able to see.

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Nodda Duma, if you write on a white paper, then it will not be white anymore. You can use white ink, but then I will not be "able to see", not even after drinking copious amount of beer.

On this topic, the Canon FD 35-70/4 lens is very interesting. I've seen a german magazine test where they rate it as best standard zoom above many other contenders (including angenieux, nikon, erc), yet it uses no exotic lenses at all, just standard cheap glasses. No barium, no lanthanum. The optical design is somewhere in the Marco Cavina website, on the page that covers the AF version of this lens (self-contained AF).