This is fairly technical question for those who know about optics in specific: Does the chemical composition of the glass elements for Leitz lens change amongst lenses of various focal length? For example, what is the chemical composition of the Noctilux 50 f1.0 vs. the Tri-Elmar 28-35-50 f4? Do slower telephoto lens differ than faster wide-angle lens in terms of the additives in the glass because of diffraction capabilities?

Alfie

-- Alfie Wang (leica_phile@hotmail.com), December 17, 2001

Answers

Alfie, many types of glass can be used in any one lens regardless of focal length or aperture. The chemical composiiton of each type of glass is what makes it unique and useful to the lens designer.

-- Douglas Herr (telyt@earthlink.net), December 17, 2001.

Yes.

But it really varies among lens elements as well. In other words the Noctilux, or any given design, can have as many different glasses as there are optical elements.

Leica spcifically mentions special "high-refractive index" glasses in the Noct. They've also used uranium-oxide (radioactive!) glass in the early 50 'crons before creating a Lanthanum glass to replace it.

Fluorite (technically a crystal, not a glass) was used in the 70's before "ED" glasses were developed in the earliest Canon 300 f/2.8s.

There is a "catalog" of optical glasses, all with different optical chracteristics, that lens designers choose from in designing a lens. I have no idea how big that list is (certainly hundreds), but I believe Leica works from a "short list" of about 50 glasses for most of their designs.

Telephotos need more dispersion correction (viz. the Canon 'fluorites', Nikon's ED lenses, Leica's APOs) especially if they are fast. Wide- angles usually don't have dispersion problems, but need high refractive indices to bend the light through up to 90 degrees on the way from subject to film.

Anyone know if Leica publishes the glasses used in each design - or is that proprietary info?

-- Andy Piper (apidens@denver.infi.net), December 17, 2001.

I have an old brochure from Leitz that shows the different glass used in several lenses ranging from the 1926 Elmar to the 1976 Noctilux. The Noctilux has two elements made from *Noctilux* glass (900403) which has an index of 1.9005. This compares to the highest indexed glass used in the Rigid Summicron of 1.6940 (LaK9). The 1969 50 Summicron uses glass with an index of 1.7479 (LaFN2).

In some cases such as the 1969 50 Summicron, the improved glass was used as much to reduce cost as to improve the lens. I still prefer the more expensive-to-make Rigid Summicron.

-- Bud (budcook@attglobal.net), December 17, 2001.

>Anyone know if Leica publishes the glasses used in each design - or is that proprietary info?<

As I understand it, they spec only the refractive index. Then they will use whichever glass they can obtain of that spec to manufacture the lens. For example, they may buy a specific index glass from Sigma on one order and then the same index glass from Corning on the next all for use in manufacturing the 35 asph Summicron.

-- Jack Flesher (jbflesher@msn.com), December 17, 2001.

"...additives in the glass because of diffraction capabilities?"

Do you mean refraction instead of diffraction? Do you have any idea what you are talking about? Are you planning to select lenses based on chemical analysis of the glass? You would have to chip off samples to do this.

-- Masatoshi Yamamoto (masa@nifty.co.jp), December 17, 2001.

Walter Mandler's 1980 Summicron 50/f2 used 5 different type of glasses for six elememts:

• LAF2 1.689
• LAFN2 1.744
• SF1 1.717
• SF13 1.741
• LAFN2 1.744
• LAF21 1.788

There are several hundred type of glasses in Schott catalog, from zinc crown ZK to phoshate crowns, lanthanium crowns and lanthanium flints borate flints, titanium flint ....

-- martin tai (martin.tai@capcanada.com), December 17, 2001.

In specifying the glass for lens element, refractive index alone is not enough, Abbe Number (reciprocal of dispersive index ) is equally important-- Abbe number is represented by the letter V.

Modern lenses are designed with computer optimization program. At the starting point, certain glasses with know refractive index n and Abbe number V are selected as initial value; during the optimazation process, the refractive indice and Abbe numbers are permitted to change. The end result of the refractive indice and Abbe number may not have corresponding glasses available; in that case, the lens designer goes back to Schott glass map ( in which a glass type is indicated by a geometrical point on Cartesian n vs V map ) and locate catalog glasses which are closest to the computed values; then followed by another round of optimization run to get the final results. It is a lot cheaper to use glass types available from glass catalog.

-- martin tai (martin.tai@capcanada.com), December 17, 2001.