Here's a summary I wrote on other forum after discussing the subject:
After producing for twenty-five years the lens that made the Leica name famous worldwide, the 50mm f/3.5 collapsible Elmar lens was recomputed at Leitz in 1951, two years after its designer Max Berek had passed away.
Max Berek’s (and Oscar Barnack’s original) concept for the very first LEItzCAmera (then called miniature camera) was designing the smallest, sharpest lens without any distortion, and a very good lens through all its range: that lens design concept was truly respected when the type of glass and the optical formula were improved. Minimum focusing distance, maximum aperture and optical design remained the same, while newer glass and small changes in elements’ curvatures and spacing improved performance by a small margin (older elmars were already great lenses) and produced a slightly larger sharpness circle and smaller softened frame corners’ areas.
The recomputing was done during the few years when lens design wasn’t a totally optical/human process anymore, but neither a computer based one: Midland and Mandler’s computer aided design work would be known only a few years later, but hoping for a shorter minimum focusing distance or for the highest sharpness paying some distortion as a price, were not part of the new recomputed Red Scale Elmar recipe from 1951: the red scale Elmar was a true Elmar, and for its recalculation both extensive visual testing and comptometers (very noisy desk sized electromechanical scientific calculators for trigonometric and logarithmic functions) were heavily used. After the Red Scale f/3.5 production, came the f/2.8 Elmar, a different lens, and one that showed a new design path in part shared with the Summicron and the rest of the more modern Leica lenses: the Red Scale Elmar f/3.5 was the last one of its kind.
During the early 1950’s the new 50mm f/3.5 Red Scale Elmar was available first in screwmount version, and then very soon in M mount… It was produced from 1951 to 1962, so the whole Elmar f/3.5 production had an impressive life of thirty-six years that started in 1926, one of the longest production periods for any Leica lens. There are two types of Red Scale Elmars: those made during the first three years of its production (1951-1953), just before the M-mount was offered, and those made from then on (1953-1962)… The first ones, with serial numbers around one million, are called “diamond” red scale elmars because there’s a diamond or rhombus figure to mark the focused distance, while the second period ones share the same optical formula and performance but are called “triangle” red scale elmars because there’s a triangle, instead of the diamond, marking the precise focused distance… It seems the change from diamond to triangle had to do with a change in the type of available glass, although performance is equal.
It was common back then at Leitz factory, upgrading cameras and lenses: many photographers sent their cameras to have them upgraded by Leitz technicians for flash use, and also sent their pre world war II lenses to have them coated, both for improved contrast in black and white and for better saturation with color film. For this reason, coating service, there are some red scale elmars which have old elmars’optical cells inside, instead of real red scale elmars’ glass and formula: to avoid confusion, those upgraded older lenses inside red scale elmar’s metal housings have no serial number, or keep the original lens serial number, while true red scale elmars have official serial numbers for the 1950’s: close to one million and above.
Now in 2017, we can say the lens was designed a century ago… It was sold from 1926 on, but it was indeed designed several years before that: Leica prototypes start in 1915-16. The most amazing fact is, both optical quality and build quality were superb, and these days, the lens keeps performing amazingly well in the hands of a skilled photographer, on film cameras and also on digital cameras…
This is the story of the small lens that changed photography forever, making it a portable hobby, and an invaluable witness in the documenting of our lives.