Sinon, Mr Putts répond aussi très directement à ta question :
"As suitable glass types seemed to be the biggest obstacle for the advancement of new designs, Leitz set up its own glass laboratory and around 1953. The first fruit of that effort was the introduction of the Summicron 1:2/50mm (collapsible). Its design was completed in 1949 and employed the new lanthanum (‘rare earth’) glass, which was being offered by the British firm Chance Brothers Glass and a new computation became necessary in 1952 when Schott glass had to be incorporated. The Summicron design was first ‘tested’ in a Summitar* disguise. Two small batches were produced, one from 812242 to 812323 and one from 812341 to 812360, both series are from September 1950. The first batch, assigned to the Summicron is from #920.000, is dated late 1951. This lens employed LaK9 glass for three of its seven elements. Leitz documentation show that the lens was designated as SummiKron too, implying that ‘cron’ might be derived from Kron-glas (Crown glass). At full aperture overall contrast is low to medium, and coarse detail is rendered with good clarity over most of the image field, corners excepted. The fingerprint of this lens is almost identical to that of the Summitar. Very critical inspection will reveal some finer differences. The Summitar has slightly better centre performance, whereas the Summicron has a more even performance over the whole image area. One might assume that the designers paid more attention to an even coverage as this lens was supposed to be the prime lens for the Leica system. Stopped down to 1:2.8 the Summicron clearly distances itself from the Summitar at the same aperture. Contrast improves, and the even coverage of coarse and finer detail is crisper. At 1:4 very fine detail is resolved over most of the image area with good clarity and very fine detail becomes visible, but with fuzzy edges, due to a trace of astigmatism and field curvature. At 1:5.6 a faint improvement in the outer zones can be detected. The lens is sensitive to flare and bright light spots have extended blurred edges. Vignetting is about 1.6 stops at full aperture and is gone at 1:5.6. " (Leica Lenses, E. Putts, p.99).

Ne jamais, jamais, oublier le pare-soleil avec ces vieux cailloux si sensibles aux lumières parasites.

A f:2 le Summicron est clairement mou, bien plus que le Summicron récent.
Quant à en acheter un sur ebay, je ne le ferais pas, ils sont tous rayés même ceux qui sont censés être sublimes ...

Achètes-en un à Malcolm, il sera cher mais parfait de chez parfait ... Note bien que je n'ai pas d'actions chez lui !
Pour le fun, voici un message qu'il m'avait envoyé à propos du Summicron rentrant que j'avais acheté ici et qui semblait très beau :

Your Summicron M, the following work in required. Minimum to service focus mount & diaphragm assy & to recoat possibly 6/7 inner lens faces, assemble then check on optical bench in order to adjust slight focus faults etc. OR the above & to repolish or replace with new damaged front lens (scratched surface).The viability accounting for the fact that its an M mount lens should be considered what ever is done.

Je l'ai revendu pour en acheter un parfait ...

En conclusion, j'achèterais un pare-soleil pour votre Summitar !