Here you open another can of worms.
Originally Posted by cliveh
An issue with some German lenses in the 1930's was the new optical glasses introduced, Leitz used them for the Summar, Zeiss for some redesigned Tessars and it went into some Novars. This glass ages badly whether it's atmospheric exposure to something I don't know but the result over the past few years is that some of these lenses have a distinct softness and despite the glass being clean & scratch free there's a very slight haziness which has a profound effect on contrast. The worst I've seen is on a Zeiss Ikonta with a Novar but I've seen it with a few Tessars and it's known in Summars. The new glasses were introduced to facilitate the production of faster lenses but they are significantly softer so more prone to issues like scratching during cleaning.
There's probably other lenses affected but those are the 3 I've seen first hand, glasses changed again towards the end of the 1930's. The 135mm and 150mm Tessars were redesigned to fit the rimset Compur shutters around 1929/30 and again by 1938 when a more modern version of the f4.5 150mm appeared with T coating.
So back to the Summar, many are soft, a great many are ruined with cleaning marks because the glass is so soft, but there are some that are OK. My first Summar came with a Leica IIA but I knew the lens was poor, that was 40 years ago. I replaced it with an Elmar. However knowing the issues with Summars I didn't buy another until I spotted a good one.
The secret to checking is the reflections in a lens, you need good sharp reflections, no haze or diffusion particularly when that reflection itself is passing through other air glass surfaces.
I've 4 CZJ Tessars (150mm & 165mm) sat in front of me 1913, 1926, 1932 and 1952/4, the worst is the 1930's version and I've had 3 that age before quite similar. My experience is the contrast difference between the earlier Tessars and the T coated Tessars is much closer than the 30's versions. Have to be careful here as not all Tessar FL's were reformulated with the new glass in the early 30's.