What does "Summicron", "Elmarit" etc. mean?
Pure Leica-speak, it refers to the lens widest aperture setting. Throughout the decades there have been a great many of these, but the most commonly used names over the last 25 years are:
f 1.0 Noctilux
f 1.4 Summilux
f 2.0 Summicron
f 2.8 Elmarit
f 3.5 Elmar ( used for larger aperture numbers as well )
And yes there are exceptions, especially with older lenses. The most obvious current exception is the f2.8 Elmar M collapsible 50mm lens for M rangefinder cameras - it really should be called an "Elmarit" but for nostalgia and marketing reasons they kept the original 1930's Elmar name (the 50mm f3.5 collapsible Elmar, manf. 1930-59, was one of Leica's most famous and popular lenses).
Apo stands for fully apocrhomatic corrected
Telyt is short-hand for telephoto
Wondering what the difference between "APO" and "ASPH" is? Ken Shipman <kennyshipman@ aol.com> posted the following clear and detailed explanation to a mailing list in July 2001:
APO stands for "apochromatically corrected". In most lenses, optical design concentrates the focus of blue light and green light into a single plane, but red light falls slightly into another plane of focus. Red subjects, therefore, would be ever so slightly out of focus compared to blue and green subjects in the same frame. Not sure you'd ever notice though, the difference is so slight. This is the same basic principle that requires you to shift the focus for infrared photography, related to the wave length of red light. In APO lenses, the design and expense has been put in to making red light focus on the same plane as blue and green. Under a microscope you would see that all light subject is now in focus, creating a sharper image overall. Many manufacturers offer APO designs, but in most of these only the very center of the lens is APO corrected. Leica prides itself on making most of the frame APO corrected.
ASPH stands for "ashperic design". Most lenses have a spherical design - that is, the radius of curvature is constant. These are easy to manufacture by grinding while "spinning" the glass. This design however restricts the number of optical corrections that can be made to the design to render the most realistic image possible. ASPH lenses, however, involve usually 1 element that does *not* have a constant radius of curvature. These elements can be made by 1) expensive manual grinding, 2) molded plastic, 3) Leica's patented "press" process, where the element is pressed into an aspherical ("non-spherical") shape. This design allows the manufacturer to introduce corrections into compact lens designs that weren't possible before. Practically, the lens performs "better" (up to interpretation) due to increased correction of the image, in a package not significantly bigger than the spherical version.
WRT to aspheric lenses, "Frank Sheeran" <sheeran@ bsag.ch> notes the following:
There is another Aspherical lens manufacture technique: an uneven coating layer is applied to a spherical lens. The coating is thicker on the edges (or on the center, depending). Canon "Lens Work II" calls these "simulated" aspherical lenses. Simulated and Glass-Molded (GMo) asphericals show up in non-L Canon lenses, while the L lenses have actual ground aspheric elements.
ASPH and APO lenses and whether they are just marketing hype was discussed in detail on the Leica Forum in September 2002. Click here to read the topic.
Wondering how the Summicron or Summilux names came about? In a thread on this topic at greenspun.com, Bob Fleischman <RFXMAIL@ juno.com> noted:
Summicron means the lens delivers the "summit" of performance in color correction, the "chro" part coming from "chroma," for color. Summilux means the summit of light transmission, for it was Leitz's fastest production lens at the time (There was a 75mm f:0.85 Summar, but it wasn't for us hobbyists). Naturally, when the f1.2 and f1.0 lenses came out, surpassing the Summilux in speed, they turned to the word root "noct," from Nocturnal, for night. Lux is not only a unit a measurement for light, but also, I believe, the latin word for light, as in "Lux Aeterna" for "eternal light."