Petri lens and lathanium flint glass

[Paul Howell]

I have collected a few Petri lens, all are very sharp for their price point. I found the manual for a Petri Flex V and the manual makes a point of saying that Petri lens were made with lathanium flint glass with high reflex index. I don't recall seeing any mention of lathanium flint glass as being special in the few books on lens design I have read over the years. Just marketing or something to it?

 

[choiliefan]

Voigtlander Lanthar is another lens made with lathanum glass. Wonder if your Petri's are radioactive?

 

[Paul Howell]

Lathanium is listed as a toxic rare earth, but not radioactive. Found some information on a telescope site that eye pieces are made of Lathanium flint glass. Nothing on camera lens.

 

[Gerald C Koch]

The fact is that the lanthanum is not going to get out of the glass. Just relax and enjoy the lenses.

 

[Paul Howell]

I am not worried about the lens being toxic, don't plan on eating them, just curious if there is any merit to the publicity hype that Lathanium flint glass was or is somehow better than other glasses. I did find additional information, seems that Lathanium is mentioned as a glass lens used in combination with other types of glasses. Don't know how common lathanium was or is.

 

[Nodda Duma]

Lanthanum oxide glasses are high index glass types commonly used in optical design today. Pioneered by Leica (the first was similar to Schott LAK-9), they have been in use since the 1950s. Often used in fast lenses (f/3 or faster). The lanthanum crowns (like N-LAK22) have dispersion similar to traditional crowns. The lanthanum flints (N-LAF7) have dispersions similar to flints.

Compared to traditional flints and crowns, they provide equivalent optical power but with shallower surface curvature. This leads to thinner lenses doing the same work. The dn/dt values, densities, and CTE values are different too. All important considerations for design work.

When you get down to it, it's just another optical material in the lens designer's toolbox. Using it or not depends on what's needed in the design.

-Jason

 

[Paul Howell]

Thanks for the explanation. other than the 50mm none of the Petri lens are all that fast, but compared to the Pentax and Miranda's lighter take smaller filters.