Actually the major problem with the Summar is that the type of glass used was comparatively soft and very prone to scratching/cleaning marks which reduced its contrast & sharpness. The Summar lens on my IIIa when I bought it 30+ years ago was awfull, I quickly bought an Elmar to replace it. Since then I've found an excellent Summar, but in reality films were nowhere near as good when the cameras & lenses were manufactured and the photographers of the time did not have our current expectations of quality from 35mm.
Originally Posted by Mark Layne
My 12" Dagor of similar vintage was factory coated after the war, I wonder how much better the Summar would be if it had been similarly coated.
Used as it would have been you'll get excellent results, but unlike modern multicoated lenses you can't shoot into bright lighting, unless you want the flare!